I agree with you and AspiringScholar here, could be a nice addition indeed! So there is actually a real incentive to collect multiple locations of a single resource, not just for the limestones.
On that note, I wonder how labor intensive simply reenabling corporations themselves would be? It seems like it was something which was just gutted out of the game, and (with the single possible exception of the briefcase icon now being changed to open an interface menu) doesn't seem like something incompatible with how the game is currently. Since all of the files are still there for vanilla Civ4, maybe it would be so simple as just putting them back in? In addition to providing an incentive to acquire multiple resources of the same sort, you could also tie that together with the RI industrial goods for a symbiosis with the mod's industrial system!
Thank you for your great answer! I obviously agree with many points, one thing I see here though is that having an archer in a stack is always a good idea (except when you're trying to put together a city assault force, and especially early on your logistics are still very limited, so every spot in that stack is very important otherwise you're going to get into pretty hefty penalties rather quickly), however they only get a bonus against recon units when attacking them, so I'm not sure if that really makes them a good safeguard against recons.
I think I would still like to see a unit get a def bonus against recon, maybe even recon units themselves?
I agree with the nerf and also that their stack aid contribution is/should be the best thing about them. In my mind in terms of game mechanics I think they should have two uses/advantages (that goes for all recon units btw, so also the later gunpowder variants):
1. In one on one situations be inferior to other comparable troops on flat terrain, but (slightly) superior on "their" terrain (in offense and defense)
2. Give stack aid contributions in the form of first strikes like they do now (maybe slightly nerf them), as well as a bonus to their terrain (so something like Recon Aid III amounts to 2 first strikes and +25% att/def on hills/forests/jungle)
Talking about and comparing the ins and outs of this game is fun, so thank you too for the discussion.
Yeah, good point regarding the offensive-only counter of the Composite Bowmen. Since they can't even outrun the Skirmishers, that's almost moot. As a ranged unit itself, it seems correct that the bonus should be defensive as well, but I am guessing that the rationale for that was because Skirmishers already have a penalty attacking cities, and archers already have a huge bonus defending them, so for purposes of city combat at least, that would make it impossible to attack them with skirmishers... but, that's not really what they're for anyway, so I think it's probably worth changing as well.
Actually, an idea just came to mind which I think would address the issue quite nicely from both a gameplay and a historical standpoint:
make Skirmishers target other Skirmishers first in combat, the way that several units already have this ability. It makes sense if they are the scouts and screening units ahead of everyone else marching and in battle, that (with their greater speed and maneuverability) they can retreat away if encountered by the enemy at a disadvantage, except against other similarly agile and maneuverable units. That seems right historically and also, in gameplay would mean that having your own skirmishers in a stack, even if not a hard counter, still provides a first screen of protection for your valuable heavy infantry, which shouldn't be first-hit realistically if such skirmishers were marching with them at fighting-readiness. This also causes their stack aid bonus to make more sense, since realistically the first strikes that they provide shouldn't be a "back pocket" ability when those units are theoretically "behind the frontline" and not directly fighting and taking direct damage. Ironically, that would actually kind of serve to make them more important, but only inasmuch as they are currently not even vulnerable to themselves, categorically. They'd also die and have to be recycled more frequently, when you can currently just keep them in reserve indefinitely as a sort of "stack supercharger" because of the aid bonus.
That mechanic definitely already does exist though, so I assume it would be extremely easy to edit.
Yeah, that is pretty much my experience as well. However the magic first strikes don't last forever though, do they? I mean the stack aid ability does reduce over time, meaning early technology recon troops + later technology other troops = lower recon aid. At least that has been my impression while playing the game. However unless I am mistaken the special promotions by some world units or national units seem to last forever, no? So bringing your centuries old Helepolis along for a siege in the gunpowder era will give you virtually no siege aid, however the defending troops are still affected by its "fear" promotion, meaning 1 first strike less. That don't seem right
Well, since the aid bonus is applied in terms of relative strength, not logistics cap (as far as I know, at least), meaning that effectively, yes, you're right, but only because the 4

becomes increasingly underwhelming as the total

of the rest of the units in the stack increases relative to them, not because less of the logistics cap is filled up as it expands with technology. As logistics caps increase, you'll likely have a greater range of unit categories in the stack as well, so indirectly it decreases, but there's nothing which theoretically stops you from just adding more Skirmishers until the Recon Aid goes back up to III. (In my games, having about 3 or 4 Skirmishers in a stack of around 15 units keeps the aid bonus maxed out even in the late medieval.)
Great idea, would add a lot of depth to late era wars.
I am kind of an air power junkie, so if this could be worked in at all, that would be really cool. It seems like the underlying mechanics are there (since riots destroy buildings from

and nukes wipe city population out already). How that gets randomized and actually applied I am not sure, but I'll gladly volunteer my efforts however I can help.
Absolutely, had the same feeling while playing. Agree with pretty much everything here, including the "just follow the leader" strategy. I don't think a complete overhaul is needed here, just some tweaks like getting rid of some prerequisites would work wonders. Also maybe it's possible to have a lower base % of technology being automatically shared through open borders, and on top another diplomatic option of "research exchange" or something where that % goes up to the level it is now? So you can choose, who you "actively" want to trade technology knowledge with.
Assuming I read the XML file correctly, I went ahead and nerfed tech transfer to 20% per civ instead of 40%. It made me wonder what exactly the idea for it was in the first place, too, because this mechanic is
already in Beyond the Sword (albeit in a slightly more limited form) from the knowledge bonus of knowing other civs which have the technology. According to the file, it's a pretty substantial bonus too.
To quote the XML directly:
And actually, I also am not sure what this 88 or the term "tech welfare" mean in the first place, but it's there, too:
So, I don't know if the "value" of 30 is a strict percentage or signifies something else, but you already have a "neighbor bonus" for tech research in the base game, so a substantial tech transfer is redundant (to my mind) for two main reasons:
1.) It already exists, and spread of technology, while naturally faster between friendly, cooperating societies, is also spurned by suspicion, fear, jealousy and such, of nations who are cool or hostile. For an obvious example, Europe received lots of technological inspiration from China in the high middle ages, proximately owing to the travels of Marco Polo, but not because of any kind of formal open-borders agreement. (Indeed, one guy traveling to another country and seeing its society with his own eyes, then coming home and talking about all of the fascinating and different things he saw pretty much exactly constitutes discovering the other team in Civ terms!) Furthermore, Europe has never had warm relations or close diplomatic ties with any Chinese state, yet it was still consciousness of their more advanced society which inspired a lot of similar inventions and technologies there. So, why you have to have a formal military access agreement for this to take place much faster seems unnecessary, even if it makes sense that it would be slightly faster if some kind of formal cooperation existed as well.
2.) On a similar note, Open Borders already indirectly boosts shared research due to increased

from trade routes which wouldn't exist otherwise. In the latter-half of the game especially, trade income becomes absolutely enormous, so, in addition to the already preexisting bonus to research for another civ you're in contact with knowing any given tech, your actual ability to research it is greatly enhanced because of your open borders relationship with that civ due to the income from trading with it. So, if you didn't have Open Borders, your research would be a lot slower, and that is mutual for the other civ too. Adding another 40% bonus on top of that seems like overkill, especially when the concept it is trying to represent is already accomplished by the interplay of existing mechanics.
So, really, what you're suggesting for the additional research agreement is basically what the Open Borders tech transfer is in the first place already! It would be cool if that could be a separate agreement from Open Borders itself, but I think most of the diplomatic options are hard-coded and so unfortunately I think that's more or less impossible within the parameters of the game engine.
Personally, I don't worry too much about enemy recon units. During the early classical era they probably are the most cost-effective unit (unless you have a city with enough food to give that title to shortswordsmen), but the scaling cost means the enemy can't build too many, and they're vulnerable to a counterattack from the rest of your stack.
For the sake of realism a re-work could be in order, but in terms of balance I think they're good how they are. And if they weren't good units in their own right their unique promotions become much less interesting, and they would be quite poor exploration units because the barbarians would kill them too easily.
I believe the stack aid is based on the strength ratio of the units, rather than the tech level directly. Not that that makes much of a difference to what you said. I do like to keep a few low-tech national units around for their bonuses - I once had a group of elite Immortals who got so experienced they were taking cities from gunpowder units!
The linear tech tree was actually a deliberate choice; Walter talked about it a while ago. He likes to force a more historical progression. I think you'd only need to alter one or two prerequisites per era to change that. I don't particularly like the idea of bringing back deliberate tech sharing - is there really any logic behind it before the industrial era? Personally I'd like to increase the basic cost of mediaeval and later techs, so that civs can only keep up with historical progress if they have lots of open borders agreements; that way isolated or isolationist countries and big, stable empires which wipe out all their neighbours would get left behind and the tech leaders won't be hitting the renaissance in the 6th century...
On the note of the tech pace, are you seriously seeing AIs hit industrial in the 6th century...?

I do notice that the tech pace tends to be a bit faster than actual history relative to the calendar date, but nothing even close to that.
But, it looks like there actually is exactly what you're looking for in TechDiffusion_GlobalDefines.xml which I referenced above, where you can adjust the entire category of tech costs by era.
While I'm making little tweaks for myself (such as increasing the Legendary threshold by 25% as CastleRum mentioned), and in particular nerfing tech transfer, I think I'll also go ahead and reenable actual tech trading, but only near the industrial era, possibly a little earlier than would be warranted by history. That's the time that tech stealing starts to take off with the arrival of significant

boosts from multiple spy slots and flat output from buildings, so the tech game gets more volatile around then anyway. Modern Physics is probably the best choice from a quasi-realism standpoint, but I think it's a bit late to be meaningful, so maybe something relatively early with a good conceptual feel like Enlightenment would be fun.
Also, if anyone else is curious, I was able to reduce the frequency of the volcano eruption and plane crash events, since they consistently fire every 10-20 turns or so in my games once their prerequisites are met. (Fun fact: the volcano eruption event is weighted at a whopping
300! when almost everything else is 100 or lower.) I turned them down quite a bit without eliminating them altogether, and I also made the volcano event something which is non-recurring. Yes, I know that volcanoes erupt more than once in history, but how often does that happen to the scale of wiping out half of a major province's entire countryside, as it does in the game?