Something I was thinking about recently was a map that would be a cliffside with scaffolding in front of it.
Effectively the idea is that you can go up/down or left/right beating the crud out of everyone.
The gimmick here is that the scaffolding
falls.
How I imagine it is that the scaffolding is divided up (arbitrarily for gameplay purposes) into an equal grid (5x5, 6x6, whatever, etc.) and as time goes on either an entire row or an entire column collapses and falls into the abyss below.
Something I think would make it easiest and fair would be that on the far sides of the scaffolding, there are ladders with platforms under them, if you fall off the ladder you might land on the platform. Climbable rigging behind the scaffolding is exposed when the scaffolding collapses but there is no platform under it at that point so if you get knocked off...it's over.