Amnesia 2

[Sexbad]

Thing is that with any sequel/prequel/etc is that you'd either have to rely on the person who bought it knowing the original story, or you'd have to spoil it in an opening scene. Neither are great ideas, especially since the story revolves around a big mystery. I'd vouch for, at most, an expansion pack, and it would have to be required for the customer to have a copy of Amnesia on their computer for it to work.