Hello! I'm a Spelunky player who has been seeking a way to make the game more interesting and challenging. Here's what I came up with:
The Companion Cube Challenge! In this challenge, you must pick up the first rock you see and carry it through the game. If the rock is lost in some way that makes it impossible to recover (bottomless pit, for example) then you must commit suicide.
(My apologies if this has been done before)