(i am considering doing a sequel)
Dont.
Leave the game for a while, come back to it when you feel like it (ie when you'd feel like making a sequel) and add another chapter and five new enemy types and more level art or whatever (or add userlevels functionality like N and build a little coffee mafia community around that).
Spend a bit of time making the AI less retarded (them not caring that you just blew the face off the guy next to them feels like a bug if anything).
It doesn't need a sequel as it stands, it needs finishing
Was pretty fun, very short, and reasonably fresh feeling. If I played it more I'm sure I'd find more holes to pick at, for now I think if you're sick of working on it, leave it for a bit then come back and tighten everything up.