But assume the party uses Rope Trick anyways and successfully hides from a pursuer. Eventually, the pursuer, unable to find any trace of the party, will leave. But if the party remains there long, wandering monster rolls are made. What happens?
First: You should definitely not rig the results of random rolls. Don't change the already-established wandering monster tables to add creatures that are specially able to deal with ethereal beings. Most of the wandering monsters will do exactly as the name suggests and wander by. The main "threat" is that the party has to judge when it's safe to lower the rope.
However, as Old Geezer put it when discussing Rope Trick on the forums, there's nothing in the spell description that says the other "dimension" is uninhabited. In the case of a Rope Trick that turns characters ethereal, there's the possibility of ethereal creatures.
I would suggest refraining from forcing ethereal results for every wandering monster roll. Again, that would be cheating. But a reasonable assumption is that someone sitting around in ethereal form will attract attention from ethereal beings, so the longer the party remains ethereal, the more risk there is.
If using one wandering monster roll per hour, assume the first five wandering monster rolls are standard rolls. Something just happens to wander by. But at the one-hour mark, and every full hour thereafter, a 6 on the wandering monster roll indicates that an ethereal creature wanders by. I'm going to use an old post about etheric beings as a guideline for my wandering monster table.
|1 or less||Efreet|
|3||Spectral Skull (no surprise bonus)|
|5||Parasitic Polyps (1d6)|
|6||Ghost Eels (2d6, 1+1 dice each)|
|7||Spirit Bug Swarm|
|8||Ethereal Jellies (2d6, 1-1 dice each)|
I'll be describing some of these creatures in future posts. Note that the Spectral Skull is actually at a disadvantage while the party is ethereal, because it normally sneaks up on a party by becoming ethereal. Similarly, you might want to waive the requirement for magic weapons when attacking a wraith or specter while ethereal.