I would love to see staircase mimics, which would have a chance of swallowing you whole if you try traveling to the next floor. I've sometimes wondered where they are in other roguelikes, as they seem like the logical evolution of that particular style of dungeon denizen.
(though there *are* floor mimics in some roguelikes)
EDIT: Thinking about this *really* makes me want to repost an idea that got lost a while back in the ideas thread...