Basically, I would love to see some type of roguelike mode for the game that's available all year round, but I think it would be really cool to have a randomly generated cave filled with mines, wild Pokemon, and treasure rooms that you get items from to then take to the surface to build secret base items. I'm thinking almost a starting area that has a few full restores or some form of exclusive item, then once you leave, you can't return unless you want to leave the instance.
A few cool concepts:
Cracked walls that lead to treasure like Big Nuggets or some other useful item.
Regi-Trio Boss fights that you can get their respective plush for after beating them.
Several underground biomes like an ice cave, mossy grove, or desert ruins
Mega Stones in rare rocks, this is simple, maybe like 1/250 odds of finding one in a rock
Mini-Events that occur like a surge in wild Pokemon of a different type, or increased odds of finding a treasure wall
Kinda huge idea for this room ik but I think it could be split into small updates as maybe a side inclusion to some events or whatever.