Changing characters' behavior in different gametypes is a terrible idea. It betrays the player's expectations and generally just leads to frustration.
Personally, I really enjoyed the concept of Circuit; the problem is mostly just that the maps tended to be designed exclusively for Sonic's abilities. Instead of maps with a variety of horizontal and vertical variation, the tendency was to make mostly flat maps that gave Sonic wide-open spaces to be fast and leave the other two behind. That also led to maps that lacked any challenge, which is boring as hell for a gametype that's all about the player-to-map interaction.
I didn't mind playing as Sonic being a thokfest - after all, being good as a character should definitely require mastery of their abilities - but there needs to be things making thokking risky. In my perfect Circuit world, Sonic players that could thok effectively through challenges, Tails players that mastered flying quickly, and Knuckles players that knew where climbing was helpful would be on largely equal ground.
I realize SRB2Riders slowed down the thok for foot races, but I didn't enjoy that mode anyway. It was still the same flat areas as vanilla circuit, just without the ability to skip past them quickly.
Also, I didn't really like how the consensus on lap length was 20-40 seconds. That isn't enough action to really do something memorable IMO, and it'd probably be a better idea to have maps twice as long with a default of two laps instead, so that races have more variety without dragging on in time.
It might also be a good idea to weight random monitor results based on position like kart racers do, so that players in the back of the pack get more speed shoes and invincibility, while players in front will get simpler things like rings and shields. (One really nice thing 2.1 did for race is making speed shoes universally faster than thokking for pretty much every character. I'm curious to see how that would affect Circuit.)
