I can think of three mods off of the top of my head that were meant to enhance the game, but not totally overhaul it. Some go a bit further than the others, but here is my list:

1. Terrae Expugnadae. Made by Lusted. "This mod aim first and foremost to enhance the gameplay and look of RTW without radically changing the underlying system. Featuring work by myself and artists such as Webbird and Warpsite, it also features night battles for the campaign"

http://www.twcenter.net/forums/forumdisplay.php?f=114

2. Bare bones war. Made by repman. "Total Conversion which combines unique strategic challenges due to a 4 Season scripted campaign from 280 BC - 180 AD on a big map and with an ruthless AI on the battlefield. It includes many new units and ideas from members of the TW community."

I dont think it is as total as something like RTR or EB. But then again, I havent played it.

http://www.twcenter.net/forums/forumdisplay.php?f=150

3. Darthmod. Made by DarthVader. "This mod uses unique ideas and greatly accepted techniques for a competitive AI behavior especially in the tactical battlefield. With the contribution of many great skinners, it evolutes to a true enhancement of RTW vanilla version game."

http://www.twcenter.net/forums/forumdisplay.php?f=71

hope it helps.

RM3