Easy, Legendary heroes are summoned to partake in the battle of your life; as the attraction is a compilation of sorts, of some of the most popular Smash Bros. characters, stages, and music pieces of all time; as Master Hand and Crazy Hand are set to bring order to the Universe. Almost like Mario Kart, but with a story and no AR.
It could work very well in a simple TF Multi-floored style attraction, with each stage incorporated via the "Stage Morph" as a part of the attraction.
Include Mario, Zelda, Pokemon, Fire Emblem, Animal Crossing, and Star Fox, and I would honestly see no reason to put in a third party. And even if they went towards the notion of third party's coming in to allow their characters into the ride, it'd be quite easy with the likes of SEGA (who controls Bayo, Sonic, and Persona).