I wouldnt mind being able to make a mount account wide.
Say we have a Hurricane at mount level 10, we can then take an item from the Cash Shop (maybe 1000 crystals, depending on mount quality/rank) that starts a quest to turn the mount into an account wide mount. Upon completion of the quest the basic skin (in the case of Hurricane, Steel Screen) would be usable for everyone on your acct, both empire and league side. Want to use Ultraviolet, or Malachite Chest? Gotta buy it on the character you want to use it on. Basic mount goes account wide at level 10, skins do not and have to be bought individually.
Heres my idea for the quest; Faithful Mount - Tested and Tried
"Hello friend, I see you have a mount there. If your interested, with a bit of training, I can help make it so anyone in your family may use your mount. Simply bring me a Mount Trainers License and a suitable mount and I can make your mount more accepting."
Mount Trainers License - 1,000 Crystals (obtained through boutique)
Suitable mount - any mount rank 3 or higher, that is at least mount level 10.