"X is definitely public domain" + "Y is definitely not public domain" =/= "The entire space of WotC works."Yes, but thst is my point: the answer To your question is "what WotC did not designate as PI." They made that decision by picking out what they were certain wasn't public domain.
Which is the whole point. Anything ambiguous has to be treated with ultra super care, and (in general) is probably best to avoid or rewrite. That's the actual reason the OGL had value: as long as you could abide by its restrictions, it let you work without fear that the edge cases, the ambiguities, the "well it might not be...but it might be..." things, wouldn't come to bite you in the butt.
When in doubt about whether a thing you intend to create and make money off of is a legal adaptation...it's best to assume the least-favorable conditions. Wouldn't you agree?