That's like saying the Tolkien estate doesn't own The Lord of the Rings because Terry Brooks wrote a blatant ripoff of it. As long as you do it yourself, you can cover all the same concepts without issue. If that includes setting changes you can make the same ones. If it involves writing code, you'd need to do it yourself from scratch, or using code that's been released for reuse. As you said (and as US copyright law also says) you can't copyright a concept. But mod ownership isn't about owning the concept (a few authors have tried to make it so, but it's not true), it's about owning the work. If someone else wants to put in the work to make a mod that does what someone else's mod already does, that's their choice, and there's nothing wrong with it. But if they take even one of the original mod's assets to do it, they're in violation. As an example, my mod adds signs and posters to the Fallout 4 settlement menus. It uses modified or custom meshes, and public domain, licensed, or custom art. Another mod can also add signs or posters (and a number do - great, more variety!). They can even use the same public domain or licensed art (minus any modifications I've made). But they can't swipe my meshes or custom art, they need to make their own.