This has been one of my favorite episodes of the Unseen64 series, partially because of the lengths that folks have gone to in the name of game preservation. Years back there was a Sonic-themed skateboarding game slated for Xbox, PS2, and GameCube from a relatively obscure developer called Vision Scape.
Built as a prototype the project ended up getting out into the wild over the years, but the story of how it met its end is rather bleak. Featuring Sonic and Shadow, much of it apes the Tony Hawk series, right down to an Emerald basically representing a Secret Tape collectible. But based on some tinkering with another mode, it's also themed around Downhill Jam's race-centric gimmick, which makes sense when it's Sonic we're talking about here. In other words, this is a pretty comprehensive mockup and even in its limited state, shows a lot of promise.
After the developer assembled the prototype and sat down with the head of Sonic Team for a formal meeting, it was agreed that they would move forward with the project after a design document was proposed. So why didn't it happen? Well, Vision Scape gave Sega the docs, but didn't hear anything back. There were no hard feelings apparently at the time, but they did feel burned when they basically took their idea and crafted their own Sonic Riders a few years later. The team was essentially screwed by way of an NDA they had signed at the start of their partnership -- Sega and Sonic Team to date have declined to comment.
There's no way to officially know what happened, but it does look bad on Sega's part. Either way I'm just glad that the prototype exists and posterity will be able to see it, and make their own decisions.