This would completely change the core design philosophy for the mount guide (and not for the better in my opinion), and mount bloat would be a problem, both for uniqueness, incentive, and server space. Breeding should be a separate system, and in fact it already is in the form of chocobo racing.

The devs have expressed that you make them aware of specific mount redesigns that you want so that they can be added (like the magitek armours, fat chocobos, and primal ponies/pigeons are all variations of each other)