so I have a theory now about Mania and Forces
it's already been confirmed that Mania is a direct Prequel to Forces
so based on some stuff I've seen in the playthroughs of Mania I belive I have some knowlage of Forces plot
"Infininite" the villian of forces is destroying the Sonic Canon by warping isolated characters through time, Classic Sonic is in Forces because infinate brought him there, and the reason Shadow and Chaos are evil again is because they are "Classic" Shadow and "Classic Chaos" I.e. Infinate brought past versions of them into the future to re-write their history, so Shadow is evil again because he hasn't met sonic yet (hopefully modern shadow will show up again at some point and not be evil)
also, it stands to reason that the new Zones in Mania are going to appear in Forces, so expect Studiopolis, Mirage Saloon, Press Garden, and Titanic Monarch to appear in Forces as well (Press Garden's achievement Blurb even sounds like it'd fit in with the storyline of Eggman being an evil dictator in Forces since the blurb says it's a factory churning out pro-Eggman Propaganda)