Okay, just the other day I cranked up Orbis because frankly I'm already bored with Civ V.
Things I'd like to see. I'm not sure how they all can be done, just putting ideas out.
* Religion: I agree the Civ IV system is weird. I personally think it works more for those of us who like our Civ more historical. I believe religion can give certain perks (like you get to build temples) but it can also be negative (going to war with a country of the same religion for example) and also I think it could be used as a mechanism for random or "historical" events. This brings me to my next point.
* Events: Keep up with the events. If there is one aspect I loved about Europa Universallis was events gave you a choice. Sometimes it was the choice between the lesser of two evils or a some bad you accept for the good you wanted. It would be difficult, but I think it would be nice to have a sort of 'event tree' where the choices you make in the game along with things like your civics and religions present would affect your events.
* National Guard: Now that Civ V has a way for cities to defend themselves and units are expensive, I really think it is time for a more realistic system about units. Standing armies are expensive! Throughout history kings went broke fielding armies. Mercenaries were used more often than not because once the war stopped, you stopped paying them. Some ideas:
-> Earlier system to buy mercenaries. Maybe tie this in w/City States. Different types have different mercs for sale. Thus those militaristic states are important.
-> Militia System: Have a 'inactive' mode for units. They cannot move and take a turn or two be become active and it cost money to do so. However it is cheaper to do then buy/build a unit from scratch and the maintenance cost is very minimal.
* Economics and War: I think there needs to be more of a penalty for long drawn out wars early in the game. Back in the day there wasn't the logistics or economic system that could fund long drawn out wars, especially since most of your fighters where also your farmers. One way to get around this for the war-mongers is through another thing you could do in Europa Universalis and that is take out a gold loan. You have a certain time to pay it back or bad things start to happen. Thus warlike cultures can take a loan but have to be careful not to go to far and may have to pillage a lot to ensure they can pay the loan back so there is still a bit of a built in check since if they pillage too much of the lands they conquer then they aren't that much better off.
* Vassal States: I live the Civ V model better. The Civ V makes no historical sense. Even if I take over an area and put in a puppet, the people aren't going to stand decades later if I decide I suddenly want one of their cities! I mean that would have been like if the USSR in the early 60's suddenly decided they wanted to annex Poland after all. The Poles, under the Warsaw Pact thumb or not, would have NOT gone for that!
Unit Upgrades: While I like it that city-states units upgrade, I find it odd other player's units don't. I've found late in the game it is 'easier' to attack another civ who still has steel swordsmen, crossbows and muskets than attack the city-state who has infantry! There really needs to be a mechanism where the AI upgrades their units better. With the importance of unit promotions, it makes more sense to upgrade a unit than build a new one.
Last suggestion is just way out there. I really think the game would be better if cities could do two things at once. In this I mean build buildings and units at the same time. I think you could work it so that if you were either building a unit or a building it could go faster but you could do both. It is really hard to play a game with a small empire strategy, emphasizing culture when another civ is spamming cities and thus can crank out endless supplies of cheap units that overrun you. Of course this relates to my point I made above about war and economics. I really don't think you should be able to field massive armies to later in the game when you have the economic (and logistical) tech to make it work. Of course Orbis can also use magic (life, nature, water magic I'm thinking) to help out.
End-Of-Line
I like this (As for the Poles I imagine if the Soviets started rounding up the Catholic Clergy bad stuff would have happened).
I think you left off a major place to morph, India. India has historically both been known for technological prowess and also its mysticism. I think it would be really interesting to do a civ with Indian roots!
(A Polish flavored civ might be cool too, complete with potato vodka )