ideas.txt

you are an earthworm, dig for treasure
you are a mole / vole, dig for treasure
you are Mulch Diggums, a dwarf from the Artemis Fowl books, dig for treasure 
  Hard to imagine how this could be fun, not a lot of movement options underground
  Would have to be puzzle-y maybe, avoid rockfalls like dig dug

you are piloting a submarine, explore underwater caves, but don't run out of air or get crushed by high pressure or eaten by a sea monster
  i like the idea of a little wind-waker grappling hook coming out of your craft
    that could get real physics-y, real fast: i'll have to see how feasible that is, make a quick demo
    alternately, use matter.js
  could I make this a little roguelike?
    grab a treasure, bring it back to the surface, get it installed, now your hull can withstand more pressure / you have a drill
    this could lead to a nice three-act thing
      tutorial, learn how to use the hook, use the hook to unlock a thing
      with the thing, more of the world is open to you, get a couple more things, letting you go even deeper
      last unlock lets you go down into some trench, where you'll encounter a boss
    

you are dreaming, and as you drift deeper into sleep your dreams get weirder. bart's nightmare?

deep into the forest, what mysteries will you uncover?

Steal Chris' idea of a game featuring fractals