For me there 2 very important things for mounts.
1. Dont get to many mounts in this game. I mean, over the time there will be more mounts no matter what, but dont overflood it please.
2. dont go the "world of wacraft-way" with high quality shop mounts and low quality effort mounts. The last years blizzard made such cool special mounts (for the shop) and really trashy or recoloured mounts for huge archievments like mythic/heroic clears. I kinda feel like , a part of mounts is to flex with them.
One simply solution to that is the fact of youp could make some customs for your mount (for example changing the colour of the skin, hair, etc...). Probably some special customs could be obtained via special quest, drops or purchased in store.