Just a game loop idea: being able to take a T1 Thrall or ordinary pet and level them up to their maximum potential. Would work very well for RP players and PvE where the journey is important. Not so much for PvP where its all about killing (its probably more efficient to just discard the lesser and go tame another desired thrall/pet).
Purge survival would help level these pets and thralls. Also taking them with the player to gain experience would help raise them, too. And if they are given the ability to harvest, then harvesting would also be another source of xp.
I’m sure specific mechanics would be worked out.