List some Killer App games that, based on their inherent strength and drawing power, could've saved a system (or kept it alive significantly longer).
Here are some that coem to mind:
Dreamcast: Messiah (This very fine PC game would've been great for the DC, as was originally planned....)
Saturn: That 3D Sonic title that Sega worked on.
32X: Castlevania (I've heard rumors that Konami intended to port Akumajou Dracula X: Chi No Rondo to the 32X, and photos printed in Gamepro magazine hinted at that possibility)
Dreamcast: Castlevania: Resurrection (Unless it turned out like any of Konami's other 3D Castlevania games, based on the review that I've read of them)
NES: Hellraiser (A Wolfenstein 3D-powered FPS on an 8-bit system!)
Nintendo 64: A worthy sequel to Mario 64.