Just to talk about GT's take for a moment though, I don't think the level design was as amazing as people tend to make it out to be. While yes it does a lot of things right, the fact that it is incredibly massive makes it incredibly difficult to know which way you're supposed to go. Personally speaking, I would get lost in Hill Top Zone for minutes on end because of how many branching paths there were with very little in the way of set pieces to keep me straight. While I do think mechanically it's going in the right direction, those mechanics mean nothing if the level design isn't solid enough to naturally allow the players to progress. Verring back into the topic though, there is such a thing as too much space in a Sonic game. While exploration is a key component of Sonic 3, Mania, and CD, 2 out of those 3 know well enough to create levels that will still funnel you to the same destination while the other gives you gigantic levels with little rhyme or reason where to go. It's probably a big reason why the boost games were linear, as the boost doesn't easily allow for that type of exploration, and honestly, I would say that's perfectly fine for a Sonic game. Linearity is not a bad thing for Sonic if balanced right.