(Dusting off an old thread here)
I like the idea of adding a Fatigue penalty for adverse terrain, but I'm not sure how that would play out, especially for spell-casting heroes (is casting fireballs while standing in mud any harder than standing on grass?).
I think that plopping "rough terrain" patches down in the middle of the field would get a bit unwieldy, especially in terms of the way the AI would have to deal with pathfinding algorithms, etc. I don't mind losing battles due to bad terrain, but I'd hate to lose battles because my commander took a different path around some rough terrain than his units!
While I'm a big fan of realism, I think we all acknowledge that combat in Dominions is a little abstracted anyway. I mean, they're individual soldiers, not units of soldiers, that we plunk out on the map in the first place. Who would really go to war with a party of 15 or 20 guys to dominate an entire province of 20,000 people.
If we really only have 15 or 20 guys fighting somewhere, why would they pick a huge field?
I think the most elegant way to include terrain in Dom 3 battles would be to abstractualize (is that a word?) the impact of terrain and borrow something from the Warlords handbook. Just have a simple grid of "Terrain Effects", where LI, HI, LC, and HC get different penalties (to Encumb, Movement, etc.) in different terrain. It makes sense to me that Heavy Cavalary is more effective in Open Plains than in the Mountains.