If they are really after a Pokemon copy they are going to have to make all the super rare one time BoP pets trade-able. Maybe they can even make up some sort of duel betting interface. Both sides put their stuff in a pot and whoever wins gets it.

Because I can guarantee you that one time only acquireable pets will not be the most powerful. Blizzard will make the most powerful pets ones that you need to sink a whole lot of time into to acquire.