Green Hill seems like an obvious choice for Sonic's home turf. However, the 'greenery' stage idea isn't exactly unique to the Sonic series, and we don't want every home turf covered in... well, turf (Mario series characters tend to have at least one green stage, DK gets foresty stages...).
I found it interesting that the music they used for that Sonic teaser video was Live & Learn. So Sonic's level could perhaps be City Escape... or perhaps Station Square... some kind of city stage. Or perhaps the ARK, at the risk of having another space level, when it's from not exactly a spacey game. I'd imagine they'd use a 3D Sonic level idea, if not then probably Green Hill.
I doubt they'd use a Sonic06 level, as that's not what they seem to be basing this Sonic off of. Otherwise, his specials could have involved the various gem moves... then again, if it was Adventure Sonic they were going for, they'd have bounce attack and fire somersault etc... So maybe, given also what is said in the dojo update, with this 'classic' simple Sonic style, they'll go for a classic level?
oh oh OH I know what it should be... Hidden Palace!!
Also, I'll be interested to hear details of his Final Smash. Evidently it involves turning Super, but will that entail just being able to fly around invincibly? With more powerful attacks? With a new superpowerful attack?
One final note... I wonder who might make an appearance as an assist trophy... or maybe as part of a stage environment...