Sega should just make Sonic Adventure 3 to appease the SA fans. I'm saying that with no love for the SA series, but I know how it is to want a continuation of a particular game/style and wait for a long time to actually get it. Mania is pretty much a continuation of the classic 2D style done right, basically picking up the spirit of Sonic 1-3&K and Sonic CD, a true sequel to those games(unlike Sonic Advance series, Sonic Rush and Sonic the Hedgehog 4, which while some are good and some aren't, feel NOTHING like the original series).
Though who knows, maybe the folks at Sega might get a hint considering how many people were chanting SA3 at the event. Obviously there's a fanbase for classic Sonic games(done right), and there is also a fanbase that would love a SA3 that invokes the feels of that series, brought to the modern gen.