Pet experience potions to level up your pet to the next stage. Only gives experience, no stats. There would be a potion for each growth: teen, adult, ancient, epic, mega. These are crown shop items.
After maximizing all of your pet stats in the pavillion, all you have left is experience. Ideally, this suggestion saves you time so you don't dislike the minigames as much. Plus, it seems a waste to use mega pet snacks only for experience when they give the best stats. Since it reduces total time playing minigames, which could be days, there is less eye strain too.
I had a suggestion for experience as well. What about pets getting xp when they actually cast in battle? There could be a limit on how much xp during a single battle or it only happens when your pet reaches ancient or epic. You're right my stats max way before i hit mega, it is a waste of snacks. Something like these would help that insanely long trip to mega. I agree totally.