Have you ever raised a raptor hatchling from legion to make it a mount?
I think that a fun thing would be to have a similar system to make a battle pet into a combat ally.
basically level it to max level, complete some objectives. ( have them out in raids, specific pet battles etc.) and you could set that pet to be a combat ally. Kind of like your class hall follower.
In combat your auto attacks have a chance to summon your combat ally for a few seconds.
They jump in, use their relevant pet battle abilities on your party/enemies and then leave and go on cooldown unable to be summoned again for a few minutes.
I really liked raising my Direbeak Hatchling to make him a mount and would like something similar for other pets