2D, no questions asked!
Unless they totally change what they have done with the 3D sonics, I would totally want the next Sonic to be 2D. Sure Sonic could be made 3D, with nice fur effects, but the levels have to be 2D!
Some of the 2D capcom fighting games nowadays have really beautiful backdrops, which I'm sure would work brilliantly with a 2D sonic.
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