If I describe the set of all small spherical furry purple creatures as "furricular", at what point does that become a useful coinage?
No. You've got it backwards.
You would coin the term, "non-fleshy humanoids". This is open set of
all people who are not humanoid and covered in flesh. It would (if they existed) include furry purple critters, and
anything else that either exist or doesn't exist and/or hasn't even been imagined yet. That is a set that is about to become useful to
us.
(This is a tortured analogy now, but it can't be helped.)
So Magical Guy starts talking about things "found" in the Amazon Jungle. "There are
small spherical purple fur-bearers!" he cries.
We say "Nonsense. There's no evidence of SSFBs."
He goes off after the next shiny bauble, declaring "And there are
large yellow rectangular spongeoids too!"
Now we need a term to throw all MG's imaginings into. The only property of the set is that it contains all medium-sized non-fleshy humanoids.
So that when he goes off with his butterfly net to look for anything that might be
colossal, green, and/or tortilla-shaped, (which are only postulated so far), I don't have to keep adding to my list of nonsense.
We can simply say
there is no evidence of "non-fleshy humanoids".
To bring this full-circle: "trans-mundane" includes
aliens,
ghosts, magic and
angels -but
just for starters. Its usefulness arises in that includes stuff we
not been talking about yet. Before the fantasy of
time-travellers popped up, it was
already included in the set
merely by dint of not being mundane. "Trans-mundane" also includes the
next woo MG comes up with (
trans-dimensional trolls) using wishful thinking and a complete lack of evidence.
Consider the nigh-synonymous term "paranormal".