Smash Bros is enough. Mario's too bizarre-o a series to really mix with anything else. But, assuming that it could somehow work in theory, crossovers aren't the best thing in the world anyway... A spin-off would work better than a crossover adventure or RPG, though, because the plot wouldn't have to be hammered out to fit both sides. Look at the Subspace Emissary--kind of a mess, leans on Kirby more than anything, a lot of the characters' actions don't fit them, the very end lets you finally pick your players because a battle between everyone and Tabuu would be ridiculous, the crazed amount of silent protagonists trying to share the spotlight doesn't allow for dialogue (but thank goodness, we could have had some spectacularly bad dialogue), very few NPCs, and much much more. But then, you don't have to give the plot a second thought for why everyone is just trying to smash each other to oblivion. They just are. And it's fun. And that's why super Smash Bros. is as wonderful as it is.