Never noticed the party drop not working. Normally you have to stay less than 6 tiles away from the player that's dealing the killing blow at the moment de ladder guardian gets mobbed. Needs extensive experimenting I guess.
Wasn't there an idea about a mob that undoes invisibility/stealth wandering randomly?
Extra idea 1 -> The mob itself is invisible (only truesight can see it), has a 1 in 49 chance of spawning everytime a new floor get generated and has huge aggro range if game engine permits it.
Extra idea 2 -> Staying on floor longer than 2 hours spawns a boogeyman which you can kill for an easter egg