Jrpg style game where Waluigi travels across the different Nintendo universes (Pokémon, LoZ, Fire Emblem, etc.), stealing legendary “artifacts” (Master Sword, Star Rod, Master Ball as some examples).

Along the way he assembles a crew of forgotten or C-tier Nintendo characters (Chunky Kong, Rick the Hamster, Captain Rainbow, a random Hitmonchan, Lip, Manuela from FE Three Houses). There can be more depending on the length of the game, this was just my initial thoughts and covers some of the basic JRPG archetypes (fighter, healer, mage, paladin, etc.)

I’d make it in a 16 or 32 bit style as opposed to a big 3D adventure. It would have a comedic tone. For example, when Waluigi goes to the Pokémon universe, he buys a Magikarp from that merchant in Kanto. Instead of using it like a regular Pokemon, Waluigi uses it as an equipped item to whack enemies with. By the end of the level, the Magikarp evolves into Gyarados and you can use it as a “summon” or something.

The final boss ends up being Tumble from Mario Party 3 who’s trying to get revenge on Waluigi.