Not a joke idea as such, but I had a very vivid dream last night which involved a 3D lemmings level and I wanted to note the ideas here.
It was a fake disjoint union with 2 halves: both halves required the lems to find their way back up to the top of the level having walked down to the bottom. The half with the exit in it also had a swimming pool which needed to be crossed.
What made the level particularly unusual though was the fact that it had
moving terrain, which activated when a certain number of lems had walked past it. So, the left side had a slope which rotated and became a series of 90 degree blocks after 10 lems had walked down it (thus preventing them getting back up easily) - such terrain could be used for easy transition back up to the top as long as no more than 10 lems walk down it.
Meanwhile, the right side featured a rope bridge which broke iff more than 10 lems were on it at once. Again - otherwise safe as long as only 10 lems are crossing it.
Finally, upon waking I thought of something which I'm sure has been discussed previously. Terrain which only Bombers can make it through, called
Blasticine. Blasticine could also be terrain which only Bombers
can't make it through. I imagine it would have an overlay which makes it look like melting wax; this could be simulated using downwards OWW, although this would obviously allow Miners and Diggers to also get through it, so those skills would have to be absent from the level.
If nothing else, "Blasticine" would make a cool level title anyway