That is actually a really neat idea, but what zones would be able to do this? I can't picture anything from Sonic 1/2/CD except for Scrap Brain/Final and Sky Chase/Winged Fortress/Death Egg. Sonic 3K is where the level transitions really stated, but many of them don't share a theme that would be able to link the levels. I could see Angle Island having accessible parts of Hydrocity on the lower water routes. Another thought would be a crashed Flying Battery in Sandopolis.