So this is how my suggestion goes:
Make each renown heart give a special map-specific currency.
Make the renown heart vendors associated with the mounts have a mount skin that can be bought with this the map specific currency.
Make the mount skin cost enough that it would take a considerable amount of time coming back every day in order to earn enough to purchase them.
Make the mount skins have at least 2 dye channels to make them more customizable than the base skins, but not as special as gem store items.
This would settle the people who want an earnable mount skin as well as giving something that would draw players back to the PoF maps.
My list of suggestions for GW2
Akkebi | Akkebyyon | Alister Kebi | Akkebi Revi | Akkebi Mememachine | Occultist Lulu | Shadow Stalker Lulu| Bonebreaker Lulu
Max Masteries | 16k AP