I like the idea of the stage rotating on a vertical plane.
Essentially, the stage itself is pretty large length wise, but it will always appear smaller than it actually is. The idea is that as long as the players are moving, the stage will rotate to accommodate them as well.
Since it is circular, there is no end to it (therefor making it work like a walk off, but since the stage rotates with the fighters it's harder to just sit at the edge of the stage and throw people in the blast zone). Being launched will move the stage as well, but if you're moving too fast you'll be KO'd before the camera can catch up with you (So if a speedster like Sonic attempts to just Run fast to KO people behind him he could end up KOing himself since the stage won't keep up with his top speed).