Since the idea of "OH____ GUYS THE BAD GUY'S EVIL CREATION DRAMATICALLY AND UNEXPECTEDLY TURNS AGAINST HIM AND THE GOOD GUYS MUST SORT ____ OUT"... has been done. Sonic Adventure 1 and 2? Good point, but you're forgetting Sonic Advance 3, of course. And Heroes. Point is, it isn't a new idea and it really does feel tired by now.
Depends. Will Sonic Next use the idea of Good Future, Bad Future, Present and Past? Will it take a leaf from Chrono Trigger's book? Or is it just one character who arrives from the future to try and kill Sonic in a 'oh wait he's the bad guy but not he's not really oh hell let's all just use the power of teamwork' twist? The end.
The idea of multiple hedgehogs has been stupid ever since Amy was invented. HAI GUYZ SAWNIK HAS A GURLFRIEND. And then Metal Sonic, because robots apparently automatically rock. And Hyper Metal Sonic because Sonic CD needs these differences. Hell, let's throw in Slitwrists the Emohog while we're at it, since Mighty the Armadillo doesn't cut it for black-and-red Sonic clones. He can be a rival for Sonic as well, seeing as we're cutting Knuckles out of that role and making him the new 'ultra-gullible guy put in charge of super-important relics' character. So we'll kill him off, and bring him back to life, and now he's like an alien. K. Oh, and then we'll have a hedgehog with a cannabis-leaf-shaped thing on his head. Because we just can. And why not a Shadow clone while we're here? Sure, he can be the REAL Shadow... or maybe not, we're not sure of that yet. But the point is, he can use giant blue laser balls or something.
Like I said. Stupid.