One thing I always like looking for whenever a game gets a big retool is the subtle or indirect way the changes will influence the status quo. With G&K, I've put some thought into how the existing civs will be buffed or nerfed by the changes in mechanics.
Note that this doesn't take into account civs like England, whose abilities were actually changed. I'm just focusing on ways gameplay changes compliment or hurt existing abilities. For example, at release, Granaries were garbage. But a few patches later, Granaries were good, despite never having been changed. Why? Because Maritime City-States got a huge nerf, so cities had to look for other ways to generate extra food for growth. Here's what I've come up with so far:
Arabia: With the increased number of luxury goods, Bazaars could become even more powerful. Currently, once you're exporting everything you have to everyone who needs it, your surplus surpluses are just sitting there. But if a greater pool of luxury goods leads to more luxuries per map and a greater diversity of luxuries within the same area (not a given, but likely to happen with some frequency), they'll be able to get even more milage out of an already bonkers UB.
Conclusion: Medium-to-Big Buff, depending on local resource diversity
Caveat: Small Nerf to Camel Archers (see below)
Caveat 2: Potential Serious Nerf if gold becomes less powerful (see "All economic civs" below)
Arabia and Mongolia: OK, I just talked about Arabia, but I wanted to put Camel Archers and Keshiks in together. It's likely that they'll both see a little bit of a decline in power based on the fact that they can't zerg units down as easily with their UUs. Of course all units are going to deal less damage, but these two units in particular will probably be hit the hardest by it. Two Swordmen can sit and exchange blows; it will take longer, but you'll eventually get the same result. But with these two UUs, you're going to have to get that situational advantage that lets you capitalize on their range and mobility more frequently, which means either your enemies are pushing further against you, or your front-line units (if you're using them) are getting beaten on more (for less damage, sure – but if otherwise they would have been able to avoid combat entirely, this is still a disadvantage).
Conclusion: Slight Nerf, still the best two UUs in the game
Greece: Some people haver claimed that Greece will be worse in G&K on the grounds that City-State favor being reworked will make City-State oriented civs worse. But I think they'll be better. Considering it will be harder to curry favor with CSes, Hellenic League could be incredibly powerful, effectively getting you twice the mileage from every point of favor you can generate. Formerly, powerful economic civs could put Greece to shame since they could just funnel unlimited money, but if city-states get that much more competitive, Hellenic League will be that much more powerful.
Conclusion: Medium-to-Big Buff, depending on how competitive the City-State favor game becomes
Siam: On the flip side, if Father Governs Children doesn't reworked, it's going to go from a pretty nice ability (but badly hurt by the aforementioned Maritime change) to pretty dismal. Going from applying to 2/3rds of city-states to 2/5ths will really suck. Hopefully they'll broaden the number of of city-state types it will apply to.
Conclusion: Significant Nerf, but possible buff if it's changed to include more CS types – may be a nerf either way if you want to focus on one type of CS, since the pool of each type will be diluted
Korea: With RA's requiring Declarations of Friendship, there will be way fewer RA's. Which means way fewer free beakers for Korea. Also RAs have been pushed down from Philosophy to Civil Service. On the flip side, if Galleas can enter ocean, Korea won't suffer from delayed ocean exploration on account of Turtle Ships.
Conclusion: Moderate Nerf (still have plenty of other research bonuses, and can still get those RAs if not as many), and Possible Buff if they can eliminate their "no ocean until Navigation" penalty
India: The rising tide of happiness (between extra luxury types and Mercantile City-States) will lift all civs somewhat, but India most of all. They'll literally get twice the extra growth potential that the luxury changes bring all the other civs.
Conclusion: Significant Buff
Denmark: The one AI improvement that the designers have harped on is naval AI. That means two things: naval civs will probably do better in the hands of the AI (on relevant maps), and humans playing Denmark will have to be much more careful when exploiting their embarkation bonuses. However, we can also now stack one warship and one embarked unit, and embarked units can defend themselves.
Conclusion: Small AI-related nerf, Moderate Buff
Songhai: Entirely dependent on whether or not River Warlord is changed. If it is, it doesn't belong in this list. If not, it's a slight relative nerf because now part of their UA has been expanded so that everyone has it. It doesn't make them any worse, but it does deprive them of a particular advantage they used to enjoy.
Conclusion: Slight nerf if UA unchanged
Rome: Holy crap is Rome getting buffed. The thing that keeps Rome in check ATM is that it's a one-trick pony. (Yeah, Glory of Rome is all right, but you usually aren't going to/don't want to/can't spam out that many buildings anyways so it's hard to leverage it into much.) But with any one-trick pony, it can do just fine if it's good enough at that one trick. Their trick is currently limited by the fact that even if you have a good amount of iron or can buy or trade for it, you're usually not going to be able to take full advantage of both Legions and Ballistae. But in G&K, Catapults no longer require Iron. Presumably, Ballistae don't either. So have fun running over everybody with your super-powered siege equipment and ultra-swordsmen.
However, with the rebalancing of unit strengths (catapults have lower ranged strength, but a higher bonus against cities), reduced damage per combat, and reworked city defense, siege weapons in general may become less dominant. If that's the case, Rome will probably still be better than it is now, but not as good as I make them out to be above.
Conclusion: Significant-to-huge buff, depending on if siege weapons' relative strength shifts significantly in G&K
Japan: Unless Bushido is reworked (in which case it's not an indirect change now is it?
), I seriously doubt it will function much differently with the new 100 HP system. Units will take less damage per combat, true, but that means that Japanese units will both get more chances to use Bushido and also get less mileage out of each use. So a little different, yes, but not really a substantial change.
Conclusion: Neither buffed nor nerfed
China and England: Gatling/Machine Gunners. I'm assuming the +1 range and x2 attack carry over on upgrade, and if they do, that's going to make these two already top-notch UUs even better. Keeping them relevant all the way through the game instead of either dead-ending or effectively losing all their promotions and UU bonuses is a huge buff. Hell, Cho-Ko-Nus and Longbows can still carry their weight against Industrial-era units just fine; upgrading them to strength 60 units with a strength 60 ranged attack (even if it's only 1 tile, or two for the Longbowman) will let them make minced meat out of anything except Nuclear Missiles.
Also, there's the fact that it seems like Archery units are being rebalanced a little bit to be more favorable than they are now when compared to siege units.
Conclusion: Huge Buff, but comes late
Babylon: If the Composite Bowman is better than the Bowman, and the Bowman stays an Archer replacement, they'll lose out on the fact that they have the best archer unit all the way up until Machinery. But it also seems like archer units in general will become a little more useful when compared to siege units, so there's that too.
Conclusion: Meh
Inca: Again, if archery units get a little better, Slingers will too. However, I believe (correct me if I'm wrong) that they'll lose out on their special trait of not taking a city attack penalty, since all archery units are losing that penalty.
Conclusion: Also Meh
Iroquois: Currently, Mohawk Warriors are incredibly good since you can mass them up while leaving all your iron for Catapults. But if Catapults no longer require iron, that becomes a much smaller bonus. They'll still be good in their own right with their combat bonus and the fact that you can build as many as you want as soon as you pop Ironworking, but as with Songhai, one of the ways they could leverage their unique traits is being given to everyone, which is a relative nerf.
Conclusion: Slight Nerf
Germany: All over the place. If Panzers are coming even later (since Combustion now unlocks Landship, not Tanks), that's a nerf. But now Research Agreements come with Civil Service, which you generally want to prioritize with Germany, which is good. But in Vanilla you needed Philosophy to unlock that anyways, so now you're back where you started.
Conclusion: ??? (but see below)
Germany and the Ottoman Empire: This one's just speculation on my part, but it seems likely that with fleshing out the upgrade paths a bit (adding Landships, Gatling Gunners, and Machine Gunners), there may be a link between Pikemen and Musketmen. That would benefit Germany, since they can spam out tons of pikemen to upgrade (albiet at an increased cost), and obviously it would make Janissaries even more absurdly powerful than they already are since you can get 20 of them the turn you hit Gunpowder, and then if you're so inclined just RA/Bulb straight to Rifling instead of backfilling while you build up Janissaries to make war with or upgrade. Both of these changes would be great, since both of these civs tend to be lower tier anyways (and we know Furor Teutonicus is staying as-is), although making the Ottomans even more dependent on Janissaries to excel would be problematic.
Conclusion: large buff for Germany, enormous buff for the Ottomans, but only if there's an upgrade link between Pikemen and Musketmen
All the economic civs: The two best ways to use money currently are to buy city-states and to sign research agreements. Those both became much less feasible in G&K. So civs like Arabia, Persia, and China will find that their money advantage doesn't net them as much as it used to.
That's all I can think of for now. What are other people thinking? In what other ways do you see the changes in G&K affecting the existing civs?
Note that this doesn't take into account civs like England, whose abilities were actually changed. I'm just focusing on ways gameplay changes compliment or hurt existing abilities. For example, at release, Granaries were garbage. But a few patches later, Granaries were good, despite never having been changed. Why? Because Maritime City-States got a huge nerf, so cities had to look for other ways to generate extra food for growth. Here's what I've come up with so far:
Arabia: With the increased number of luxury goods, Bazaars could become even more powerful. Currently, once you're exporting everything you have to everyone who needs it, your surplus surpluses are just sitting there. But if a greater pool of luxury goods leads to more luxuries per map and a greater diversity of luxuries within the same area (not a given, but likely to happen with some frequency), they'll be able to get even more milage out of an already bonkers UB.
Conclusion: Medium-to-Big Buff, depending on local resource diversity
Caveat: Small Nerf to Camel Archers (see below)
Caveat 2: Potential Serious Nerf if gold becomes less powerful (see "All economic civs" below)
Arabia and Mongolia: OK, I just talked about Arabia, but I wanted to put Camel Archers and Keshiks in together. It's likely that they'll both see a little bit of a decline in power based on the fact that they can't zerg units down as easily with their UUs. Of course all units are going to deal less damage, but these two units in particular will probably be hit the hardest by it. Two Swordmen can sit and exchange blows; it will take longer, but you'll eventually get the same result. But with these two UUs, you're going to have to get that situational advantage that lets you capitalize on their range and mobility more frequently, which means either your enemies are pushing further against you, or your front-line units (if you're using them) are getting beaten on more (for less damage, sure – but if otherwise they would have been able to avoid combat entirely, this is still a disadvantage).
Conclusion: Slight Nerf, still the best two UUs in the game

Greece: Some people haver claimed that Greece will be worse in G&K on the grounds that City-State favor being reworked will make City-State oriented civs worse. But I think they'll be better. Considering it will be harder to curry favor with CSes, Hellenic League could be incredibly powerful, effectively getting you twice the mileage from every point of favor you can generate. Formerly, powerful economic civs could put Greece to shame since they could just funnel unlimited money, but if city-states get that much more competitive, Hellenic League will be that much more powerful.
Conclusion: Medium-to-Big Buff, depending on how competitive the City-State favor game becomes
Siam: On the flip side, if Father Governs Children doesn't reworked, it's going to go from a pretty nice ability (but badly hurt by the aforementioned Maritime change) to pretty dismal. Going from applying to 2/3rds of city-states to 2/5ths will really suck. Hopefully they'll broaden the number of of city-state types it will apply to.
Conclusion: Significant Nerf, but possible buff if it's changed to include more CS types – may be a nerf either way if you want to focus on one type of CS, since the pool of each type will be diluted
Korea: With RA's requiring Declarations of Friendship, there will be way fewer RA's. Which means way fewer free beakers for Korea. Also RAs have been pushed down from Philosophy to Civil Service. On the flip side, if Galleas can enter ocean, Korea won't suffer from delayed ocean exploration on account of Turtle Ships.
Conclusion: Moderate Nerf (still have plenty of other research bonuses, and can still get those RAs if not as many), and Possible Buff if they can eliminate their "no ocean until Navigation" penalty
India: The rising tide of happiness (between extra luxury types and Mercantile City-States) will lift all civs somewhat, but India most of all. They'll literally get twice the extra growth potential that the luxury changes bring all the other civs.
Conclusion: Significant Buff
Denmark: The one AI improvement that the designers have harped on is naval AI. That means two things: naval civs will probably do better in the hands of the AI (on relevant maps), and humans playing Denmark will have to be much more careful when exploiting their embarkation bonuses. However, we can also now stack one warship and one embarked unit, and embarked units can defend themselves.
Conclusion: Small AI-related nerf, Moderate Buff
Songhai: Entirely dependent on whether or not River Warlord is changed. If it is, it doesn't belong in this list. If not, it's a slight relative nerf because now part of their UA has been expanded so that everyone has it. It doesn't make them any worse, but it does deprive them of a particular advantage they used to enjoy.
Conclusion: Slight nerf if UA unchanged
Rome: Holy crap is Rome getting buffed. The thing that keeps Rome in check ATM is that it's a one-trick pony. (Yeah, Glory of Rome is all right, but you usually aren't going to/don't want to/can't spam out that many buildings anyways so it's hard to leverage it into much.) But with any one-trick pony, it can do just fine if it's good enough at that one trick. Their trick is currently limited by the fact that even if you have a good amount of iron or can buy or trade for it, you're usually not going to be able to take full advantage of both Legions and Ballistae. But in G&K, Catapults no longer require Iron. Presumably, Ballistae don't either. So have fun running over everybody with your super-powered siege equipment and ultra-swordsmen.
However, with the rebalancing of unit strengths (catapults have lower ranged strength, but a higher bonus against cities), reduced damage per combat, and reworked city defense, siege weapons in general may become less dominant. If that's the case, Rome will probably still be better than it is now, but not as good as I make them out to be above.
Conclusion: Significant-to-huge buff, depending on if siege weapons' relative strength shifts significantly in G&K
Japan: Unless Bushido is reworked (in which case it's not an indirect change now is it?

Conclusion: Neither buffed nor nerfed
China and England: Gatling/Machine Gunners. I'm assuming the +1 range and x2 attack carry over on upgrade, and if they do, that's going to make these two already top-notch UUs even better. Keeping them relevant all the way through the game instead of either dead-ending or effectively losing all their promotions and UU bonuses is a huge buff. Hell, Cho-Ko-Nus and Longbows can still carry their weight against Industrial-era units just fine; upgrading them to strength 60 units with a strength 60 ranged attack (even if it's only 1 tile, or two for the Longbowman) will let them make minced meat out of anything except Nuclear Missiles.

Conclusion: Huge Buff, but comes late
Babylon: If the Composite Bowman is better than the Bowman, and the Bowman stays an Archer replacement, they'll lose out on the fact that they have the best archer unit all the way up until Machinery. But it also seems like archer units in general will become a little more useful when compared to siege units, so there's that too.
Conclusion: Meh
Inca: Again, if archery units get a little better, Slingers will too. However, I believe (correct me if I'm wrong) that they'll lose out on their special trait of not taking a city attack penalty, since all archery units are losing that penalty.
Conclusion: Also Meh
Iroquois: Currently, Mohawk Warriors are incredibly good since you can mass them up while leaving all your iron for Catapults. But if Catapults no longer require iron, that becomes a much smaller bonus. They'll still be good in their own right with their combat bonus and the fact that you can build as many as you want as soon as you pop Ironworking, but as with Songhai, one of the ways they could leverage their unique traits is being given to everyone, which is a relative nerf.
Conclusion: Slight Nerf
Germany: All over the place. If Panzers are coming even later (since Combustion now unlocks Landship, not Tanks), that's a nerf. But now Research Agreements come with Civil Service, which you generally want to prioritize with Germany, which is good. But in Vanilla you needed Philosophy to unlock that anyways, so now you're back where you started.
Conclusion: ??? (but see below)
Germany and the Ottoman Empire: This one's just speculation on my part, but it seems likely that with fleshing out the upgrade paths a bit (adding Landships, Gatling Gunners, and Machine Gunners), there may be a link between Pikemen and Musketmen. That would benefit Germany, since they can spam out tons of pikemen to upgrade (albiet at an increased cost), and obviously it would make Janissaries even more absurdly powerful than they already are since you can get 20 of them the turn you hit Gunpowder, and then if you're so inclined just RA/Bulb straight to Rifling instead of backfilling while you build up Janissaries to make war with or upgrade. Both of these changes would be great, since both of these civs tend to be lower tier anyways (and we know Furor Teutonicus is staying as-is), although making the Ottomans even more dependent on Janissaries to excel would be problematic.
Conclusion: large buff for Germany, enormous buff for the Ottomans, but only if there's an upgrade link between Pikemen and Musketmen
All the economic civs: The two best ways to use money currently are to buy city-states and to sign research agreements. Those both became much less feasible in G&K. So civs like Arabia, Persia, and China will find that their money advantage doesn't net them as much as it used to.
That's all I can think of for now. What are other people thinking? In what other ways do you see the changes in G&K affecting the existing civs?