Long time ago I started working on a Paper Mario & Sonic crossover. Featuring these 4 plus Mario, Luigi, Peach, and Donkey Kong as playable characters.
Basically their worlds collide, and Bowser and Eggman team up. With Bowser being spiky and Eggman being used to being hit by spiky hedgehogs, they both perfectly counter the other’s rivals.
Robot Koopalings, Deadly Six with clown cars and magic wands, and a lot of minions being combined.
A reoccurring baddie trio would be Wario, Waluigi, and their newest member, Rouge. They would hunt for treasure and she’d wear overalls and a hat.
As far as combat goes it would play like an rpg, you’d have 2 characters at a time and could do combo attacks.