I've got two:
-I say it a lot, but the level design in the main campaign is generally fine aside from many of the levels being too uphill focused, giving Tails and Knuckles a speed advantage over Sonic. If Sonic is to be perceived as the fastest character then the levels should be design with a more primarily horizontal and downhill mentality with hidden goodies and slower pathways for Tails and Knuckles to reach that Sonic cannot.
-I feel like a lot of issues could be fixed by simply making more use out of the Spin button in midair. For example, Metal Sonic's hover could be nerfed and moved over to Spin so that his main double jump action could be something more speed focused, maybe a variation of the drop dash that transitions into "running" on the ground instead of spinning. This would keep some of his mobility buff over Sonic without him necessarily being too overpowered. Then when you go Super the running drop dash could transition into full dash mode when you land and he could get the OP hover back.
Likewise, moving Sonic's thok over to spin would leave him open for a move that has more vertical mobility and is safer for new players to use without taking away the usefulness of the thok as a momentum and jump correction tool for advanced players. Maybe Sonic's double jump action could be changed to a double jump that lets you slow fall with a skydiving animation for a few seconds if you hold the button down? Then when you go Super you still have the double jump, but the slow fall gets replaced with hover.
I realize that anything that involves keeping thok in on Sonic is definitely going to be a big hot take at this point though, which is why I listed it here. I think drop dash, even an alternative variation of it will be one too as I recall the drop dash getting a big solid "no" from the devs at some point.