This is something I miss from WoW. Basically, a string of achievements such as:

(Collect 5 Mounts)
(Collect 10 Mounts)
(Collect 25 Mounts)
(Collect 50 Mounts)

...and so on. Same for minions, with more tiers than just 50 and 100. Then add a pet or minion reward for each achievement. I'd suggest rarer/unique mounts for the harder collect achievements and to make it so it's obtainable ingame without cash shop or discontinued items, but still very difficult to hit them all.

Basically, it's an extra incentive to collect. There's no better way to show off your collection than riding around on the collect 100 achievement mount and it gives you an ingame goal that has the collector actively seeking mounts and minions.

We already have this ingame with things like triple triad and orchestion rolls and it's great. So why not expand on it further? Give us collectors more endgame.

Cheers