If it were to happen I won't see how story wise. Other than continued pursuit of the Phantom Ruby. But if Sonic Mania 2 will have the same gameplay as the first, graphics as the first and same story as the first... then why bother with a sequel at all?
I mean sure, same can be said for Sonic 2 on the Mega Drive but Tails was a major game changer.
That's why I think that Sonic Team should of made "Level Pack DLCs" for Sonic Mania Plus. Simply an assortment of stages to play through for no story or reason other than fun.
If it were up to me ....
Change the characters, that's how it will affect on gameplay.
Dish Mighty and Ray,
replace them Amy (with hammer like in Sonic Advance 1) and 1 brand new character that has new gameplay.
Also replace Knuckles and Tails with Vector and Charmy.
Knuckles and Tails still could be unlockable later after finishing the game once
(just like on Sonic CD remake).
And make 2 player stages that could be played in the same screen, not split screen.
This one is kinda hard, but if we're talking about sequel of the best Sonic game of decade, then aim big !