I think that SE should feel free to bring in new mounts but only if they are going to add a substantial amount of lore around how, why and where they are used as mounts. If you look at the Grand companies page and try to imagine it with soldiers mounted on a diverse number of different mounts the army would start to look more like a disorganised mob rather than a well organised militia. So i hope if they add more mounts that they do it slowly and try to be sure that they wont look too out of place together.