I'm very interested in the excessively complicated idea of making a repeating cave in trap for demons. You'd need a freezing biome like a glacier (because why the Hell not), but basically you just allow water to flow into an exposed tile a little bit at a time, it forms a floor because there's less than 4/7 water in the tile, and it collapses because the tile is surrounded by bridges/floodgates rather than proper walls and floors.
But, having built all that gearing and stuff, you could reliably cage trap demons. Without manually reloading the trap each time. Which would be nice. I originally thought of it for FBs actually, but demons are basically the same thing.
Z LEVEL 3
---
~B-
---
Z LEVEL 2
----X-
-+BH.X
----X-
Z LEVEL 1
--------
----CCC-
---D++CP
----CCC-
--------
- wall
+ floor
. open space
X floodgate (always closed)
H floor hatch (always closed)
~ water source
P pressure plate
B bridge, raising on z2 and retracting on z3. Both linked to the pressure plate
D a door or any other furniture
C cage trap
All aligned to the left of the blueprint. The extra space in the water airlock thing is so that the 7/7 water spreads out enough to only be exposed as less than 4/7. Also that open space on z2 has to be exposed to the sky, else the whole thing is an excessively complicated waterfall decorated with cage traps.
I have yet to embark on a freezing biome and actually enjoy it long enough to get this built though so if someone would like to do this and report their findings I'd be quite happy