I'm most curious to see what's next for Classic Sonic. I have a suspicion that we're gonna see a new collection of the classic games.
And realistically I think it'll look something like this: Sonic 1, 2, 3, Knuckles, and maybe even CD, all in HD wide-screen. 3 and Knuckles will probably have to have different music if a new deal can't be worked out with Jackson's estate, maybe beta tracks or new tracks from the guy that did the Mania music. Each game could now have spin dash and drop dash could optionally be enable. Maybe allow for Tails and Knuckles to be playable in each title as well. Base these on the mobile versions so that Hidden Palace Zone is put back on 2 and bam you have a definitive collection for these old games. This all seems pretty realistic, though maybe CD wouldn't be included.
If they wanna go the extra mile, upgrade the graphics of each game to look on par with Mania and maybe even include Ray and Mighty as unlockables. I could maybe see parts of this happening.
If they wanna blow our minds though: include a remake(demake?) of both episodes of Sonic 4 as a single title so that it also looks like Sonic Mania graphically and shares the same proper classic physics we know and love. Adjust level designs as necessary for fluid gameplay and consider bigger changes like if homing and the team attack should remain. This ain't happening, but I'd love it, it could fix Sonic 4 into an at least decent title and stop the structure of the series from being so odd where Mania looks more like Sonic 4 than the Sonic 4 we did recieve.