It's probably too much to ask for but it would be pretty cool if we could get Mario as an unlockable character, maybe locked behind beating Pipe Towers.
I do really hope to see Dark City Zone. My imagination goes wild thinking of the possibilities we could see implemented in the level design. Starting out at street level, eventually working our way indoors and outdoors again a few times, moving between multiple different buildings, maybe even one of them could be a museum of some sort with all kinds of crazy exhibits built into the level design. Eventually end up onto the rooftops, jumping from building to building, maybe have Act 2 be raining. It would especially be cool if the Brak Eggman boss was more of an escape sequence where you are effectively doing fast paced stealth while finding projectiles to launch at him without being seen.
In regards to Red Volcano Zone, something I would love to see is a chase sequence in Act 2 where you are running away from a huge T-Rex badnik. Not have it take up too much of the Act, but just like a portion of the level somewhere in the middle. Then for the boss, you get to fight it properly after it manages to catch up to you and corners you. Maybe also have a small portion of Act 2 contain a snow and ice theme in reference to the scrapped Blue Mountain Zone?
I would love to see the gimmick where the player's size changes return in Egg Rock Zone. Maybe even have the boss fight take place while Sonic and friends are huge, with the fight being a rematch against Brak Eggman.
Lots of wishful thinking here, but dreaming isn't a crime.