Mainly mean from a hardware perspective. But can be other reasons.
Magical Truck Adventure is a Model 3 game and looks stellar and would have been a great game to have. Maybe a US launch game or even if Japan only. I don't know why it wasn't.
Planet Harriers. Man, this feels like the perfect Dreamcast game. I never got to play it but I want to.
Zone of the Enders or should I say Enders Zone. I know it began as a DC game and I know why it was moved to PS2 but I'd prefer it on DC. SEGA consoles and mecha action games went hand in hand.