I just thought of this. It would be like the egg hunt and squishy hunt quests, and it would go under quests, because it IS a quest!
Except this time, it's a quest for the alternate Res-verse, where pets that are stuck in limbo go.
So, for the first few tasks, we have to gather supplies. Simple things like a rope, maybe a weapon to defend yourself, etc., gathering them three at a time. Maybe tasks 1-10 could be like that. Then afterward, you journey to a cave you believe to be the site of the entrance to the alternate Res-verse (it makes sense for this to be Relcore, though there could be slightly different versions for each planet if you'd like, to spice it up). During the travelling, some of your items break, and you have to find new ones. Occasionally, you may be ambushed by wild Creatu, and that makes your selected Creatu's health drop by a random amount. If that occurs, you must heal your Creatu before you move on to the next step.
Finally, you get to the cave (say around level 20-25), and you can't see into it. For this, you have to obtain a candle. When you get the candle, you journey through the cave, being attacked, items breaking, etc. until you find a vortex that takes you to the alternate Res-verse.
By this point, you have beaten the game, and it starts all over again! Maybe with a "This was all a dream" type of feel to it.
Naturally, if there will be a different quest for each planet, then different Creatu will appear, your selected Creatu will have to differ (need a swimming one for Atquati, flying one for Scria, etc.), and the journey overall may differ, but this is just a basic outline.