Here's some "world premier" footage of an unreleased Dreamcast game called Boarder Zone / Supreme Snowboarding 2! It was sent to me by the game's artist, Miha Rinne. It started out as a straight port of Supreme Snowboarding from the PC but ultimately the devs added so many upgrades and new features to the game that it became a sequel. Sadly the Dreamcast version never came to be but it was eventually released as Transworld Snowboarding for Xbox.
The game was at a playable state on the Dreamcast but sadly none of the devs I've spoken with so far have a build of it. I think there's a good chance it can be found though as the developer, Housemarque, is still around.
Edit: You can see some artwork from the game on Miha's portfolio:
https://www.artstation.com/artwork/AqgloN