There was a NES and later SNES prototype (By Pete Warnes and Carl Graham) that was IIRC the focus of the 3D chip discussions. X was cool, and a good selling point (and predated the NES work), but there was (again IIRC) no interest on going further with 3D on GB. Seeing custom hardware on carts (Pilot wings, Mario Kart?) and having dealt with the custom hardware on the Konix, gave Jez a good avenue to explore with Nintendo.