Light-as-Kira and the Light who's lost the Death Note and therefore his memories of it somehow meet. Because Light is a massive narcissist, and because I'm curious about whether the memory-less Light would have any sort of crisis of conscience upon learning about all the murders he's committed from an outsider's perspective.

(I'd be thrilled with any sort of Light/Light, honestly; you don't have to stick to the Kira-Light/'innocent' Light concept.)

(Incidentally, this is an amazing idea for a ficmeme.)