I think it's fine to add characters that turn Sonic's physics-centred design on its head and make it about something else while navigating the same levels. Of course, this makes level design exponentially harder, and I loathe lock-and-key as much as anyone. But it's useful to think of additional characters not in terms of what they can do that Sonic can't but in terms of what they can't do. Amy in Sonic Advance is remarkable because she can't curl up into a ball, not because she can hammer down springs. Knuckles' best moments in S3&K are the ones in which you'd have benefitted from Sonic's high jump, which he can't do and, because of that, you must take another path and you must use his abilities. Sandpolis Act 2 is the best example I can remember. It's an asshole, but it's great exactly because you get fucked for not being able to play they way you're used to. So if extra characters were based around arbitrary challenges (play as Charmy and you'll die if you get hit, regardless of how many you rings you have; play as Espio and, I don't know, being targeted by badniks fires off an alarm that swarms the level with more threats; play as Mighty and you can never kill badniks or get item monitors but will only be able to bounce off them) and then get them extra abilities could make it work even if the gameplay is no longer about physics.