Age of Empires 3 introduced horses pulling all the carts and siege equipment. The siege weapons also spawned a crew which operated and moved them around once unpacked.
It was a really neat detail that added much to the overall feel of the game that seems to be missing both from the trebuchets firing in the trailer as well as some packed carts further back in the mongol base.
Could these make a return? The self-moving and firing siege equipment clashes with the otherwise realistic look the cities and units (albeit exaggerated for visibility) seem to be going for.