I agree, there are myriad ways in which terrain could be used in battle.
It's not entirely far-fetched to consider that some terrain might block line of sight/targetting for ranged units altogether. For instance, i'm assuming that walls do this - preventing ranged units from hitting units on the other side of them. It's entirely possible that they do not, but it would reduce the usefulness of walls. At the least, as you said, walls could grant a defensive bonus to those units on the other side of them.
I suppose we won't know until we see.
A few other ideas: Being at the top of a tall incline could increase ranged unit's range instead of increasing damage, but increase melee unit's damage or armor. It would probably still increase their movement speed as they went down the hill. This would give people incentive to put ranged units ontop of walls (if they can shoot over walls from behind. If not, they you'd have to put them on the walls anyway).