One definite improvement that I would like to see is in sieges... Although I wholeheartedly loved MTW the sieges were pretty bad which wasn't a problem as you fought 95% of the time in the field.

In Rome, we had much more elaborate sieges but you had to fight them all the time which proved to be a long and tedious process. One primary reason was because you'd get stuck on walls fighting enemies who are supposed to be running for their lives but still standing there fighting it out with you to the death. I've had peasants holding off heavy infantry for 5-10 minutes simply because fighting on the walls meant only a few people could engage at the same time.