That's essentially the same thing as modding a game, though. I know there have been lawsuits to stop modding, but I don't think any were successful.

I'd challenge you here to think about this in terms of the legal aspects rather than reaching specifically for similarities as similar is often meaningless in the law or contracts when specific acts are codified rather than generalized ones.

I'd say what we're talking about here is probably a fair bit different to modding a game in most aspects.

I haven't followed any relevant cases but I would be surprised if there's any serious dispute that the common methods of modding games generally create derivative works. I think the dispute would be downstream of that as to whether or not the mods are covered by fair use.