They tried a partnership too long ago and it didn't worked out. It's always a problem when they have to decide how much each one will get from the profits, so that becomes a no-go.
Even putting the characters could be a problem, because the other company will want a level of creative control. Let's just say that Sony won't want that Drake does X or Y thing in SSB and he can't be a slouch that just got beaten easily, etc.
I remember Marvel vs Capcom. In the first game, they were balanced. In the second one, the Sentinel, Blackheart, Shumma-Gorath and Dr Doom were massively overpowered. It always seemed like some Marvel influence saying "oh, this guy has to be strong, it's not good like it is, blah, blah" and here we go.












