The Sonic 3D in 2D fan game is a fully complete project, developed with a mix of Fusion 2.5 and Sonic Worlds Delta, along with original development/customization from Sotaknuck.
This fan game, titled Sonic 3D in 2D, isn’t a wholly original work, but it does have a compelling hook. Sotaknuck’s game takes the general storyline and level locations driving the strange Sonic 3D Blast, and re-imagines it as a 2D sidescroller.
Drawing a lot of inspiration from Sonic 3 & Knuckles, this interpretation of the game re-positions it as a sort of sequel to the games before 3D Blast.
You can check Sonic 3D in 2D out for free on Sotaknuck’s itch.io page, with also includes installation instructions and a larger feature and content breakdown.