Shenmue II got released on Dreamcast in Japan.
I have the Gold version of Half-Life for Dreamcast. Imagine everything from the PC game being there, but having to wait every 60 seconds for textures to load no matter where you were.
That reminds me of the Tiger Game.com games that were announced right before the system died. There was a WCW game - I think only a screenshot of the "select wrestler" menu exists.
I remember Madden, Metal Gear Solid and Castlevania: Symphony Of The Night were also announced.
Almost forgot my favorite unreleased game ever:
Hellraiser 3D for NES
Apparently the developers had a smooth enough frame-rate, but the chip technology they built into the cartridge would have made the game upwards of $100 for consumers.