Isn't this just the Sonic collection?
No, it's a new game with a new story and bosses and stuff except most of the zones are from previous games.
But the old zones are completely redesigned and have new features and things.
Even zones that didn't make it into the game got some level parts re-skinned and added to others. (like marble garden zone's stuff in stardust speedway)
There's also minor things like the Blue Spheres Plus and Mean Bean Machine minigames, but that's all extra.
Sonic Mania: Milking It because Sonic Forces flopped edition
The people voted with their money and Mania won. Why not give people DLC?