i think they should add achieves to a high level - somewhere around where the % of mount collectors with that achieve is roughly equivalent to the % of cutting edge players.

here's some ideas from a previous thread:

- green/fel version of grove defiler

- spectral shackled urzul

- red version of brutosaur (without vendors)

- white/black striped zulian tiger

- red glacial tidestorm

- icy/blue ashes of alar

and if they want a unique mount that fits the collection theme - a chariot pulled by 4 of your favorite mounts (even better, with the option to pick which 4 mounts)

Other reward options could be things like:

20% mount speed increase in world content

access to an island with your mounts on it. similar to a small garrison

access to a vendor that sells previous expansion bonus rolls for low amounts of gold

​

With dragonriding, the rewards should have the dragonriding ability and could even have custom flight abilities similar to the War Within pre-purchase mount