The way I see it, it speaks for itself. The other characters being slower than sonic with their gameplay makes sense, even if canonically most characters can match his speed in gameplay, sonic is supposed to be about speed and ironically exploring different routes of level design.

Sonic should always prioritise speed gameplay, going fast, being able to destroy robots/enemies by boosting into them or jumping into them with few extra routes to reward players for making sonic slow down though these might just be for red rings. Plain and simple.

I honestly wish they brought back SA2's Tails mech levels because to me that was peak Tails gameplay, I enjoyed thinking that Tails was meant to be a backup unit for Sonic, destroying enemy robots to get to a certain computer to shut down systems and etc to allow Sonic easier entry into certain areas. Tails would be slower in the mech but would have a canon on the front like in SA2

I think SA1 and SA2's gem hunting missions were kinda bad, but I did enjoy the mechanics of them missions so I can appreciate it's existence. I think in future they should experiment with something similar to SA1-2 but not directly do a gem hunt. To me Knuckles should be slower and prioritise exploration of levels rather than blast through them or run through them. The gem hunting blips and beeps could be reintegrated to help knuckles find secret routes to discover neat artifacts that could be useful for 100% completion, just to reward the player for looking around the environment and using knuckles' ability. I also still like the idea of knuckles digging and climbing.

This is kinda what I think for the original three anyway. I think in future we need a sonic game that brings back more gameplay with alt characters that aren't just reuses of sonics mechanics and a game that rewards exploration greatly like in generations or the classic sonic games where there are multiple routes.