I think it would be awesome if you could spend ap to do stuff like that, but each different army would have to have their own thing. The humans and the undead could have horses, fine, but what about the Wolfen? What could they get? Caltrops? Traps? Nets?
That's another thing for the devs to think about, animate, and more inportantly, BALANCE. Right now, with the game still incomplete, their current focus should be on getting new units and races balanced, not adding onto the already existing ones.
All in all, I would like more customization, horses or otherwise, but this kinda goes along the lines of something to think about once the game is like 80 or 90% done. Once that rolls around, there are some other things they could so, but that's another subject for another time.