I had an idea for Wolf Dungeon event a while ago and thought I'd share:
Nona get butt-pregnant from some bug or smth with lotsa eggs/larvae.
Then camera switches from regular Nona to Top-Down view of her intestines and you now control one of those larvae in the BATTLE ROYALE. You gotta control your larvae to collect food (to grow,get stats and get stronger) and beat up your competitors/siblings to become the Top Slug.
Nona picture is to the side where you may see her struggle with bowel movement, eating food to compensate for the food eaten by larvae, getting larvae/worms out of her butt by pulling them out or trying to poop them out. Food may have different effects and Nona sideview can give you heads up for these or for hazards that may get you out of her system before you're fully grown. For example Nona can gulp a lot of water which will create a "flood" that gonna pull you "down", so you must get to the "wall" and stick to it.
When you reach full growth it is 1v1 versus fight event. Could be your slug against your brother slug. Or it could be some entirely different monster parasite and this whole madness was just to remove it ( in which case your slug might as well get a hero cape ). When you're done, Nona poops you out as an arm-sized slug.
I think it should be slug so its an easter egg, cuz there was iirc elukus game before Fairy Fighting where you play as a slug, and it was an easter egg in Fairy's (Ana Moguri stage? idr). Hell this whole thing might be triggered by a literal easter egg that Nona can find.