I imagine that the "difficulty levels" would essentially be names for the areas; if so, a few ideas off of the top of my head:
(These might be a little cliche - I'll confess that I'm tired, and so may not be judging these as well as I might. ^^; )
A very difficult level involving lots and lots of dakka: Leadstorm
The first level, little more than a path from one point to another: Point A to Point B
Another easy level, attempting to reach a cake at the end (a real one, for preference :P): Cakewalk
A difficult level involving either lots of traps or lots of opponents faced each singly: Gauntlet
A first particularly difficult level, representing a noticeable jump in difficulty, or a difficult level involving either much shooting or much fire: Trial by Fire
A level involving a tricky puzzle that entails repetition with small changes on each pass: Iteration
A level involving a puzzle that entails the use of an object's effect on the object itself (such as using mirrors to point a laser at the laser emitter), especially if this intensifies or modifies the result: Recursion
I'll leave those for feedback, I think, rather than continue; I don't want to keep at it if these are not what you're hoping for. ^^;
My Games & WritingThose are pretty good.
I have thought of a few that are pretty much references to other games and things.
Just being me.
I was thinking on make some kind of Fanfic/Writing Project based on the trope Idiosyncratic Difficulty Levels. Basically a Mad Scientist has created some kind of experimental facility where it gets harder and harder each time you go up. And, there are hundreds of difficulties rather than just: "Easy", "Normal", "Hard" these difficulties are more precise than the ones I've just mentioned.
And, I need more ideas. That's just thought. So?
Just being me.