Most interesting Saturn prototypes/unreleased games?

As someone very deep into the hobby, it's been fascinating to see all the beta builds and even completely unreleased games that've come to light in recent years. I also enjoy reading the stories of how some get preserved, which sometimes requires nothing short of a heruclean effort by the community (and/or finances!). Much respect to those devoting their time so gamers like myself can enjoy the end result.

Prototypes by system/Sega Saturn

I've messed around with the various Sonic X-Treme protos & read about The Sacred Pools (not sure a puzzle FMV is for me... cool story tho!).

What else is out there that you've found interesting? Whether it's a pre-release build, an unreleased game, anything that wasn't a typical retail release.