If we go with a non-scrolling screen, we can have only one section of the level visible at the beginning, and as the player kills enough enemies/unlocks the right doors, etc other sections of the level would be revealed. And returning to certain previous sections may be part of the level design... but that's going off topic.
The revealing of sections would need to be graceful, not just a sudden appearance. Maybe some sort of gradient fade in/out.
I'm going to mark this resolved for now. I may re-open it later, but I think the levels work pretty well as they are now, and we've got doors that can move between levels. I do like the idea of returning to a previous area though...I'll add that in.