Sonic superfans The Great Lange and Murasaki Fox have whipped up a prototype for a fan-made game called Sonic Utopia and it's got a heck of a trailer.
As you can see, Sonic Utopia reimagines the classic Sonic gameplay in an open-world 3D terrain. It looks kind of glorious, albeit in an early state. After all, The Great Lange created all the models and animation from scratch, along with most of the textures, sound and music. The engine too is proprietary, with Murasaki Fox taking credit for that one.
The mix of open-world exploration and suggested paths carved out in trails of rings suggests something like Mirror's Edge or Gravity Rush wherein you can explore at your own leisure, but the landscape will be peppered with checkpoint races that put your traversal talents to the test.
It's unclear if Sega will allow the project to continue, but in the past the publisher has been supportive of fan endeavors. When Sonic fan Christian Whitehead (not to be confused with Eurogamer regular Dan Whitehead or staff writer Christian Donlan) released a proof-of-concept YouTube video for his proprietary Sonic CD port, Sega actually hired him to make an official re-release on modern platforms - one that turned out quite well.
You can download the Sonic Utopia prototype here.