I personally would like to see more the mega drive/genesis era levels, because, in my opinion, they were the best and truest Sonics. The games that made Sonic a household game and basicily secured the megadrives hold on the console market and inspired a generation of "Mascot Platformers" after it.
Personal fave levels I'd love to see done in Generations style are:
Sonic: Marble Zone, Star Light Zone and Scape Brain Zone
Sonic 2: Mystic Cave, Oil Ocean, Wing Fortress Zone And I always loved Hilltop zone.
Would also see like a boss run mode where you'd have to do all the classic sonic bosses in a row but remade with "shiny" graphics.