Hmmm, stage ideas, let me think of something.
A big exoctic rooftop of a building with glass ceiling on some areas(in this case is the floor, since you're on the ceiling)., and like a big heliport n the middle, the glass parts are transition to lower roofs of the stage, each transition would lead to a different room all together, with like, one leading to a big corridor, the other to some business room with a big table, one with a huge Server, with a huge Nobreak filled with batteries as well, that would be an interesting danger zone, The default room of the stage, the ceiling that i was mentioning would have walls on all ends, and maybe some of them eletrecuted for more danger zones?
Throwback stage, a big lab with a bunch of giant test tubes, but instead of having just naked kasumis all over the place, you could have more people (clones of all the ninjas), the floor and walls being danger zones as well, and also, some screens on the roof and the walls that would show scenes of past games of the characters that are being cloned.
A stage actually inside the Mugen Tenshin clan, a stage with a lot of transitions and slopes, maybe with a rainy variation, story reasons, MIST invades the mugen tenshin clan with their army of phase 4..
A big, dumb, happy amusement park.
A really cool snow stage with transitions, blizzards and slippery floor.