uh...... what is "crossover"?
if you mean cooperation between different companies, there are already old posts talking about it
you may have to edit the topic if you mean different things
Mario and Sonic, and NOT in an olympic setting. I want to see the italian plumber and speed demon hedgehog stomping on Goombas and smashing robots.
A Mario RPG where Bowser brainwashes the other playable heroes from Super Smash Bros. (Pit, Link, Kirby, etc.) as part of his new plan to take over the world.
Mario and Donkey Kong that's different from the Mario Vs. Donkey Kong games.
A four-way cross-over where Mario, Link, Kirby, and Samus end up in each other's dimensions and have to figure out a way back home.
Mario and Mario. A Mario cross-over featuring the mainstream games and the RPGs. And for the entire Mario series to make a cameo.