@pluralistic A reason I've heard why, without the OGL, you wouldn't be able to create adventures, classes or other material for D&D, is that they'd be a derivative work. Like writing an 8th Harry Potter book.
But I've always felt that that would only be true if you make stuff that actually takes place in their established game world. I understand I can't publish my own sequel to their campaign, for example. But shouldn't I be able to publish my own adventure for my own game world (or independent from any specific world)?
I think I should, but many people say I can't. Of course in that adventure, I'd also be referring to various monsters, spells, and classes. Are those part of the unprotected game mechanics, or part of their protected trademarks and copyright?
I don't know. Nobody seems to know. I think this is something that needs to be tested in court, but nobody other than Hasbro has money for that.