I don’t think graphics are inherently that important to an RTS but there are some CPU heavy components that could greatly enhance gameplay experience. Some of the things I think that should be included:
Persisting corpses, with a corpse counter in the options settings.
Somewhat higher population limits (250?)
Being able to combine units into divisions, so for example you could put 10 crossbows and ten pikemen into a division, and they would all be rendered but they would move coherently and you wouldn’t have to reselect them every time.
What else do you think AOE IV can do to take advantage of more powerful modern hardware?