Xenoblade Musou (Warriors)

Edit: Not really a crossover, but it was on my mind for some reason.

Since they've already worked together, why not a Mario + Rayman platformer?

I still want my Sonic/Mario RPG'ish crossover where Sonic and Mario's worlds collide and Bowser and Robtnik team up to take them out

This makes so much sense, we can only hope that now when their Olympics-games are seemingly done a project like this is closer to being real

Sadly, It prob will never happen. There would be too much conflict on who gets creative control between SEGA/Nintendo i think

Hm, didn't think about it in that way, maybe why they went the sports-game route in the first place. Well, we can still hope.

@spemanig Capcom vs. Nintendo

Mario and From Software

I've only wanted Nintendo Vs. Capcom for years, Nintendo, get on it already!

As others have said though, Sonic and Mario teaming up against Bowser and Robotnik would be sweet. Probably won't happen though, and if it did, it would never be as cool as Super Mario Bros. Z, if anyone remembers that.

Throw in Mega Man and Dr. Wily as well and I think we have something here