With BTS I think almost everything needed is added. I would like to see all the elements from all of these added, with a couple of things being fixxed. For one, if a city has a certin religion, you must be able to atleast have the area where the holy city's religion uncovered. As far as I know, Christians have always had an understanding of where Jeresuelum is. In the game, I might have no idea where the holy city is. Another problem is Vassal states. These need a major overhaul. I think it is obvious what the flaws are, but I will list a few anyway. If you demand a technology, they shouldn't be able to refuse it, or else there should be war. Same goes with money, or resources, etc. They should not be allowed to spy against you, and should have a few less abilities (not being able to found corporations, build wonders, etc). In return, you should have quite a hassle if you have a vassal civ, espicially if you have multipule. You should have extra maintiance costs for vassal civs that you as the master must pay. You should have to actually protect them, for instance, if another civ takes on of their cities, you should recieve a large unhappiness bonus in all cities with that's civ's ethnicity if you took over the civ, depeding on the size of the ethnic population. If the civ was a friendly vassal, you should have a moderate unhappiness occurance in all your cities. And finally, civs should be allowed to cancel vassalization. It perhaps for gameplay balance, shouldn't be able to happen immeadiatly, but as with canceling vassalization, there should be a minimum time requirement. Another couple of things, while easially editable in the XML, I still think the defult should be changed. There really should be an added percentage to all defense, and another additional percentage to defense on own territory, and another larger percent for defense on cities, and units should be produced faster. Several units take as long to build as various buildings. These should be revieved by all units. And my last major qualm with Civ4, the game speeds. Quick games do not play or feel the same as marathon games at all. The main cause of this is the movement costs. If a quick game is 4 times the speed of a marathon game, then units in a quick game should have 4x the movement allowance. I can see the problems that may arrise from that, specifically the fact that if you have a very large military, you could possibly move in and take over a whole country in a single turn, but I am sure there is a way around that, which can be figured out by some thinking and creativity.
A few things I think should also be added.
Civs comming up in the middle of the game. The revolution mod takes care of that, but an officialized version, allowing revolutions, barbarians starting civs, the colonies has been handled in BTS. I also like how the revolution mod allows for leader succession in some form, however I think Firaxis could do something more creative. Perhaps when changing the goverment civic you move to a new leader, and when you enter a new era. There should be a variety of traits for various different leaders of each leader, so when leader changes occur, there could be a major shift in strategy nessissary. This however may be hard, considering you must decide the personality of each leader, and artists make new heads and animations for each leader, it clearly leads to a much better game (I feel). I highly doubt the creators would do such a thing, but I think it's a good idea. I would also like to see things that have the name in, but no real implimention be reworked. For instance, inflation could be government controllable to an extent, and could have positive and negative effects based on how much and how certin things are happening. I would also like to see a larger role played by trade routes. Tech points should be transfered through trade routes. You could possibly gain access to a new resource through trade routes behind another civ's back, but you'd have to pay extra. Trade routes could effect ethnicity of a city, and there are a number of other possible things. I also think ethnicity of a city should have a large effect, however I don't know what. Clearly it should effect chance of revolution, but what else I don't know. And I'm sure there are other systems which I think would be great that I'm not able to think of right now.
Also, Civ4 is a major resource hog. People have found ways to speed it up, and I don't understand why Firaxis didn't do that from the begining. Now, I understand there is a limit, but what optimizing that can be done really should be done. This will allow people to be able to play into the late game with the vastly superior large and huge maps on marathon speeds without being victim to 5 minute load times between turns. If anything would really really be good for the next civ game it would be this. To allow a next generation game, to run on this generation's systems okay, and on its own generation's systems very well.