If they wanna keep the gimmick they used with Sonic Mania and Sonic Forces, I can try and predict their next moves.
Main Sonic games always release at the end of the year, around november, so i predict a new 3d sonic game for november 2021
Now, it's gonna be an anniversary, so they clearly don't have only that game up their sleeves...given the success of sonic mania, another 2d sonic game is definitely in the works and they'll probably give the mania team a bit more budget and (hopefully) allow them to make 100% new zones. It's probably coming out around summer 2021
But, anniversary thing and all, they've probably been working on an adventure remake, or at least a heavy remaster...one can only hope they're making a collection with both adventure games but whatever...this game will probably come out before the others...probably at the beginning of the next fiscal year, after march 2021...they could do it this year in time for christmas, but I don't have any hopes for that.
So the games would be: adventure remake/remaster, new 2d sonic and new 3d sonic. What about the gimmick I said in the beginning? I believe they'll somehow connect the plot of all these games. Just like they did in Mania/Forces, they'll probably bring back some plot points from the adventure series that they'll add in the future 2D game, just to let it come full circle in the 3D game when they'll bring back the goddamn classic sonic.
I would have added closure to the whole Infinite and the Phantom Ruby plotline, but apparently Sega now hates it so much they don't allow them to be in the comics or either in the Sonic mobile game.
I may be totally wrong...we'll see...