I don't think SA2 Sonic/Shadow Levels were all that great, they were linear and all about timing and doing "Tricks/Stunts" right. I prefer Adventure 1 which played more like the old games, where each stage was different in geographical design and gimmicks, in SA2 every stage was essentially "Speed Highway", and the setpieces were more repetetive,and there wasn't really anything to explore. Unleashed was linear and all about fast too, but was more about avoiding traps hazards etc. then tricks which feels better for a Sonic game IMO, then SA2 way of doing things in a way. Still I agree that 30 stages of Speed levels would make a great game. The Thing that ruined ruined SA2 to me was the Treasure Hunting tough, Eggman/Tails Levels weren't what I call fun, but they were at the least inoffensive. Knuckles and Rouge's levels gave you the bubonic plague. I've had visions of the eternal flames that were more pleasant and less psychologically torturous. I still remember flying around for 60 Minutes in Meteor Herd as a Kid. What a Nightmare. What for alot of fans is the game,with which they became a Sonic Fan,is for me the the first "love/hate" type game I experienced as a Kid. I loved Sonic/Shadow,Chao Gardens and Story, rest annoyed me.
Anyway about Sonic Generations...waiting more info.