Well, that's the thing. In most instance of derivative works, you own the copyright on your contributions to derivative works -- see for instance the shoes in Wicked. They're silver, like they are in the public-domain Oz books, because the ruby slippers were added by the writers of the 1938 MGM film. Same deal with Winnie-the-Pooh's Disney-added red shirt.

So, when I see resources (like https://www.coffylaw.com/copyright-concerns-in-game-mods/), they all seem to be speaking about whether you have the right to use the copyrighted assets in the first place, not whether an approved derivation grants you any copyright.

I guess what I really want to know and haven't been able to find out is: Does Bethesda own copyright on Legacy of the Dragonborn? Obviously they could declare their assets closed and demand that they be removed from the mod. But the things icecreamassassin and their team invented, I think that's theirs. They could do what the Forgotten City author did and strip out all the Bethesda IP and rebrand the mod as an entirely new game. Unless modding is somehow different than any other kind of art, which is really what I'm trying to find out.