That's already in the game, it's called "Time Attack Mode." But seriously, once you get all the Time Stones in that Sonic CD Restored mod, you unlock a menu option to always start each level in whichever time period you want. That's kind of close! But yeah, I've always wanted a "Time Locked" mode where you can play the entire game in any time period. Though I guess that would require tweaked graphics for the third stages to have Past and Present variants.