I would like to see a move toward bringing back glorious achievement goals, as were seen in MTW. Conquest is fine, but it can get boring when you have taken most of the map, and the fall of other empires is inevitable. I can never finish a conquest game for this reason.

Also, I think it would be better for rebellions to take place if your garrison is too large. Or at least, for large garrisons, possible vices for the generals should be geared towards oppresion, and hence a decrease of happiness. I think it is better if there was some sort of balance. This could be helped by giving titles to natives rather than the conquerors.