During the events of Sonic Mania, The dimension is accidentally different. This dimension, there's a Sonic that can speak after Sonic Generations. Also the timeline has been altered due to Amy Rose joining Sonic, Tails & Knuckles. Amy always has a crush on Sonic, but when he see's him going into the portal, Amy has no choice but to follow Sonic whereever he goes. That also triggers Classic Eggman to go in as well, along with Classic Tails & Knuckles.

But when going into the wormhole, Classic Tails and Classic Eggman accidentally finds themselfs into Equestria. What does Equestria has to do with any of this? While Eggman thinks he defeated his enemy, the Main Six and the Cutie Mark Crusaders are in this Universe so it's back to plain old destruction.

Future Mobuis, during the events of Sonic Forces. Classic Sonic and Classic Knuckles find themselfs where Modern Tails is fixing E123 Omega. Where's Classic Amy? Classic Amy accidentally finds herself in Modern Eggmans base and has gotten captured.

Everything goes acoordingly planned in Sonic Forces, minor thing is that Classic Amy and Knuckles join the battle. Does Classic Tails, Eggman, Main Six, Cutie Mark Crusaders, Button Mash, Rumble Lane, and Shady Daze have anything to do with the main battle?

You'll have to wait and see!