Total impossibility, but I once made a roster anyway.
I think it would be amazing, the roster would have like 60+ characters, 50+ stages, 100+ items, the intros and outros of Nintendo characters, costumes of Nintendo characters instead of color, A mix of ring-out and super, like people take a whole lot more damage before being able to be knocked off screen and people will still be able to use supers if they don't want to sit there and spend 5 minutes beating somebody up. I would also want Nintendo to b ethe one to make the game, because I find all the DLC in this game bullsh!t (unless all DLC was going to be like how Kat and Emmett was.)
Nintendo character would also have combos (or nobody has combos and we go to the casual gameplay of SSB) and a level 1 and 2 super that I would love to see.(3rd super can be when they get the smash ball.)
Plus the story would be epic.
The title could be "Super Smash Brothers Battle Royal"
The Project Axis Actually do it : http://pastadium.forumotion.com/f14-project-axis