Did the aliens from The Next Generation episode Schisms ever return? It seemed like they were probing this universe/dimension, and not for friendly purposes. Do they make any other appearances in other episodes?
From Memory Alpha (emphasis mine):
Director Robert Wiemer, Brannon Braga, and Michael Piller were all disappointed at the look of the aliens, and decided not to bring them back, despite the open ending. Braga stated, "I felt they looked like monks – fish monks – and monks aren't terrifying." (Star Trek: The Next Generation Companion)