I'm trying to find an FPS game for the Dreamcast that I haven't played.

These are the Dreamcast FPS Games I know of.

Commercial:
Quake III Arena
Unreal Tournament
OutTrigger
Half-Life
Tom Clancy's Games

Ports:
Doom
Heretic
Hexen
Hexen 2
Quake
The Rise of The Triad
Wolfenstein 3D
Marathon 1
Marathon 2
Marathon 3

Homebrew:

Thank you.