The game already has depth. Adding more terrain modifiers could be fun, but not necessarily. I don't think the game terribly needs it, at this point. Derek emphasized positioning in combat, and that adds some strategic depth.
Right now (I think Frogboy said this) tactical isn't so much about battle decisions as it is about expresing the consequences of your larger strategic decisions, such as research, city count, faction choices, etc. And that works well, for me.