depends on the derivative. I can't make a story about hogwarts. But I can make a story about a girl with a scar who gets enrolled in a magical academy. And I probably just described 20 works with that description. But I think the legal definition of "derivative work" isn't that loose.

The more gray areas for derivative work is stuff like fan art, translations, and remixes. All are technically in the area to be copyright'd, but at the same time it also helps publishers in essentially being free, natural advertisement.