I would think if Nintendo did a Sonic game, they'd probably try to keep it as much as the 2D games as possible, but with 3D. We'd get more Sonic and The Secret Rings like games, which would have more gimmicks, more platforming, and a bigger game in general, but also much less speed. It would get boring, really.
The 2D department would be pretty good, though. They'd probably play like the old games, but with more acceleration, an air speed dash move, and hopefully an actually decent jumping engine.(seriously, jumping in most 2D Sonic games is a pain in the ass )
Platforming with speed in 2D works. Platforming alone in 3D games works just fine, as well as speed alone. But the both of them won't work. You'd have to slow the game down everytime you have to platform.(that, or feel the wrath of unplayable-ness) It would ruin the pace of the game, and that sucks.
If they want to get 3D Sonic games to work, they'd have to mix speed, action, and adventure to get it to work. Platforming doesn't play the biggest role in the Sonic games. Small things like rails and jumping over huge gaps without losing speed would work, but not stuff like precision jumping. Would you rather want to wait for moving platforms to come down and do some platforming instead of just shooting yourself to the action with a couple of springs, or getting carried upwards by use of a tornado or giant whale, or perhaps even running up at superspeed?
Right, some guys would rather want to do the moving platforms stuff. Thats what Mario is for. Once you do it with Sonic, it takes the 600 miles a hour to mere 4, and thats not fun. Remember Sonic's level Crazy Gadget? Right, thats where Sonic and extreme platforming(and puzzles) mix.
Anyway, except for the spin-off games and perhaps 2D games, I don't think Nintendo will do any better than SEGA. We'll have better quality Sonic games, but of the cost of speed and crazy action, and there's no good Sonic game without them. If SEGA would just try the Sonic Adventure style Sonic games again, and put some actual time in the gameplay,(not 1 and a half year) we'll have another great game.
