Actually I remember seeing screens of a GBC FPS. It had some weirdo name like
Dinosaur Harry or somesuch. It looked slighly lower tech than
Wolfenstein 3-D on the PC. I assume it either proved unfeasible (though that's a relative term given the fact that
Dragon's Lair was ported to the system) or took too long and was buried in the wake of the GBA.
Schnapple
http://members.tripod.com/schnapple99/