I would love for Nintendo to ask Sega if they could develop a Mario and Sonic crossover platform game...and then Nintendo puts a premiere development team on it and creates the BEST 3D sonic game with a great sense of speed, camera angle and the game feels like sonic.
That too me would be the ultimate Nintendo does what Sega...don't?
Which is why Sega would never allow that to happen.
Well Nintendo already showed that they could in Mario Galaxy. The parts where you become Rainbow Mario have Mario moving quite fast and when he jumps he does a summersault similar to Sonic. Not to mention did it all with an amazing camera system as well.
If Nintendo were to have a Mario and Sonic crossover, I'd imagine Sonic would control just like Rainbow Mario, except faster. Which would make him Rainbow Luigi then.

This is why a Nintendo Mario and Sonic game developed by Tokyo EAD would work out great. Mario could remain the same as he is in Mario Galaxy, while Sonic would play like the Rainbow powerups, except with different moves to seperate the characters. This way they'd be able to play the same levels, except their different moves would allow them to reach different area's in the level. It would end up similar to Sonic 3 where Tails and Knuckles can both access area's that Sonic can't.
And since it'd be developed by Tokyo EAD, you know they'd be able to design each level in a way so that they take advantage of both characters strengths and weaknesses. In other words, you'll have fun playing through the game as both Mario and Sonic.
Of course, the only people allowed to develop this game are Tokyo EAD and
ONLY Tokyo EAD. Iwata should give an order to security that any Sega or Sonic Team members that come anywhere within 500 feet of the studio are to be shot at sight, no questions asked.