My Sonic Project 2017 guesses (based on gut instinct and general buzz): The game will essentially be the Sonic 2 equivalent to Generations. No major gameplay changes, but there will be refinements (see how Generations changed from Colors) and new moves. I'm guessing a similar Act 1 (2D) Act 2 (3D) layout. Sonic Team could potentially bring back Classic Tails, introduce Classic Knuckles and Amy, and have them playable in the 2D stages, while the 3D stages could do the same with Shadow, Silver, Knuckles, Tails and others.
As far as story, my guess is a Days of Future Past style story which kicks off just as Generations did with a playable level – but this time as Modern Sonic in a apocalyptic setting. Like Unleashed, we could be thrown into the midst of a battle, but this time Eggman is about to take over the world and cause serious damage. Perhaps Sonic travels back in time (a la X-Men DOFP) to stop Eggman and teams up with Classic Sonic, going through time to build up a resistance against Eggman for a final battle. No throwback stages, everything is brand new, but perhaps (like Sonic Mania) we will see retro-inspired settings. Story will likely be darker, to appeal to the SA2 crowd.
A year and a half is still a ways off, so I think they are testing the waters to see how people reacted to both Classic Sonic and the dark setting, seeing which way people want the feel of the story to go.
