Once stamina is added properly terrain could easily be more important. Things like cavalry not being able to charge through forests/swamps, or long distance marching lowering stamina so charges become difficult/impossible. With roads allowing long distance travel without affecting stamina it would provide more tactics and strategy. There's the option of making terrain even more interesting by making ranged units have a shorter range in forested areas if it's not too complex to add. Or formations can't be used in forests. I always find in games that the more you can use terrain to your advantage the more fun it is. If walking along a road, or standing in a forest, makes no difference to the battles then all it will be is eye candy (and resources in the case of trees). What could/should be added out of the many possibilities is the main question though! I'm not sure whether being able to build roads would be used enough to warrant adding. I think I'd prefer them to be already present on certain maps. As long as all maps have a few routes that are good for marching then i think that would do.