Tangents
No Super Sonic button - Fault carried over from the classics for no reason.
Framerate - I don't mean the 35FPS cap(though 60+ would be great), I mean it's easy for the game to stutter with skyboxes or slightly large/detailed enviroments. OpenGL usually fixes this, buuuuut.....
OpenGL barely supported - We can emulate Software's "retro" style, yet for some reason we still main Software when OpenGL has more possibilities. It's got better performance on today's hardware, and when there's more options on what you can render on-screen, there's more creativity for developers, both for vanilla SRB2 and mods.
CEZ3 Boss - Boring fight where you wait out the boss for eons. Plus, Tails/Knux can cheese it altogether by waiting on the outter ring.
Metal Sonic Boss - Awkward stumble where you "fight" Metal as slowly as possible to avoid bouncing into the electricity or death pit. Then you wait eons for him to invincibly spaz around. It drags on.
Character Balance - We balance Match/CTF stages to have exclusive sections for Tails and Knux, and we even threw out Circuit for them. Yet in Single Player, they make the campaign easier than even Super Sonic. So here's my suggestion: if we accept Tails and Knuckles will make the campaign easier no matter what, we can focus on buffing these characters into ACTUALLY being equal to Sonic in competitions, where balance matters.
We COULD just nerf Sonic, but then we have a slow unfun game that only lets us go fast when the stage is specially designed for it. It'd be similar to forceskinning Knuckles in the old Circuit maps. Or watching a marathon starring the elderly in wheelchairs. Or like droplets of paint on the wall racing to dry the fastest. The thok's the only way to sneak speed into ANY level no matter the design. And I don't see why Tails or Knuckles have to be so excruciatingly slow. Ideally we give every vanilla character an FSonic speedboost mechanic, and leave Sonic's thok exactly as it is. Everyone will be fast and thus fun to play, but Sonic can go fast instantly while the others can fly.
With everyone pretty fast, I'd suggest to increase the size of the attacks in Match/CTF to compensate. A bigger bombring explosion, a growing bounce ring, an autoring firing 2 rings side-by-side at the same ring-cost, a larger red ring, and the rail ring can get more ammo.