I haven't played MPII full or demo yet, but I''m downloading the demo right now. I'm sure I will like it.
Anyways, I think to add a little bit of replayability without having to model more and more full levels. Add something like a "free fight" mode. Just hand players some guns and let them run around shooting insane ammounts of enemys, maybe even allow them to unlock a couple of new things like more weapons and stuff.
Just a thought.