I did actually originally plan to make a flying horse, although it would be more along the lines of replacing the horse's hooves with thaumostatic devices. The plan was abandoned because the harness and its associated mechanics (haste enchantment, girdle, etc) already fill the flight niche pretty well - making a flying horse would either render them obsolete (if it was faster or didn't require fuel) or not be worth lugging an animal around.
Since the Endersteed is the only mutation species that can be mutated into a more advanced species, I would like to suggest another mutation mutation. The Guardian Panthers are a great way to enhance barely useful ocelots. Considering their size I was thinking it would be wonderful to further mutate them into a ridable variety.
This one is a much-requested feature, and I'll definitely try to implement it in a post-1.2 update along with other creature meta-infusions.
All pocket planes technically spawn in the same dimension in order to minimize resource use, but they are separated by opaque and unbreakable barriers so that the player does not notice.