My idea would be exploring a cave with several floors. The game
wouldn't be about trying to find stairs to lower levels, but instead
finding and fetching items the expedition group asks from the cave to
the surface. The emphasis will probably be more on exploring and
avoiding hazardous environment rather than beating brains out of cave
bats, though I'll add some weapons in, too. I don't think the game
will have very wide variety of items, though: mostly stuff you really
need like flares, bombs and ropes (helmets?), and the artifacts and
items you bring to the surface.
My biggest problem will for sure be the lighting and the procedural
part, but we'll see how that turns out!
To be honest, no. This wont be a platformer, and instead of level-
based score-earning, you'd explore always the same, bigger cave with
several layers. Also the emphasis would be more on careful exploration
rather than running under a time limit, as in Spelunky.
I can't deny that there will probably be something similar between the
games, even if I didn't plan so.