I haven't made any changes or improvements to it in a while, but in terms of core gameplay changes, Assimilation can be fun (or at least I think so, since I made it for my own use
).
It lets you recruit a small selection of a nation's units and commanders at their capital once you've captured it, which gives a bit more incentive to conquer:
http://forum.shrapnelgames.com/showthread.php?t=46696