would be good for forcing innovative gameplay. I'm imagining 'crimson room'-style clue-finding, moving walls, lots of small traps to avoid, that kind of thing.
Alternatively you could compromise and restrain the player inside a 512x512 room, but allow other things to move in - monsters teleporting, whole bits of scenery, etc.
For a more silly application, I once made a speedmap that was kinda space invaders/asteroids themed. It trapped you in a box and scrolled spaceships and asteroids by you, and you got a score based on how many of them you hit. I'd be interested in seeing someone do cool things with that idea.
http://www.quaddicted.com/filebase/sm66_pack.zip
if you're interested.
Also, I should speedmap again.