If 2d Classic Sonic was given the storyline/plot of Forces, I honestly could see Sega hinting more to characters and franchises seen in the Classic Sonic era. We already seen Fang, Bean, an Bark being cameoed in Mania, so I would assume they would be back as full characters for Sonic to interact with. I could see Robotnik having references more to Satam and Archie Robotnik as they center around him conquering Sonic's world, and I wouldn't doubt if one of Robotniks' croonies from the older shows (Scratch and Grounder or Snively) made cameo appearances to fill in Eggman's current two bumbling robots. I am not sure if Sega would hint at the FF, though, seeing how Sega doesn't like to tie in to the older Sega of America characters. If the FF did make an appearance, however, I could see Sega taking the risk of making them the extra playable characters that would replace the fan-made character seen in the modern Forces game and with them an element of gameplay change. Sonic (and the FF if Sega dared to take that risk) would have similar gameplays that the characters in Sonic 3 have (fast running speed, spin jump, spindash), but like Sonic 3, the characters would have different gimmicks that would allow them to interact with the environment differently and either find hidden goodies, complete specific missions, or discover secret pathways. If this was the case, then the wisp gimmick would be thrown out of the game (wisps and classic Sonic don't go hand to hand). I could also see 2d Sonic finally having a scripted story instead of non-narrative cutscenes in the older titles, and the scripts would probably play more with either the American characteristic of Sonic or his more quiet self seen in Sonic CD.
Modern Sonic and Mania? I don't think I would see much of a difference with this and any other boost game. I doubt Sega would take the risk of trying a new formula like Lost World or Boom at this point, so the game would probably be similar to Sonic Colors in game mechanics. They would probably keep to the same story outline seen in the 2d Sonic game, but I have a feeling they would use scripted cutscenes which means all the characters would behave as their modern appearances. This means that poor Knuckles would be back to a bumbling idiot instead of the island smart hero we see in the 2d game. I also could see Sega only making Sonic as the playable character to focus again on the boost mechanic meaning Tails and Knuckles are back to be supporting characters. Modern characters would also probably have some cameos to replace Fang, Bean, and Bark (most likely Shadow, Blaze, or Silver...maybe even Big). In summary, a 3d Sonic Mania would probably be similar to Sonic Colors in gameplay and plot, so the game probably wouldn't be as big of a deal as the 2d Mania has proven to be.