>Thus, the user files were “substantially similar” because they functioned as sequels to the video game itself—specifically the story and principal character of the game. If the user files had told a different story, with different characters, they would not be derivative works. For example, a company offering a Lord of the Rings game might include tools allowing a user to create their own character from scratch. If the user used the tool to create a hobbit, that character might be considered a derivative work. A unique character that was simply a 21st century human in jeans and a t-shirt, not so much.

Not necessarily - the game engine code is also independently copyrightable separate from the story the game is going to tell. Writing a completely different game using that code - even if you just distribute level files - might still be an infringement. The analogy given by the EFF also doesn't match and is subtly wrong. The unique character would still be made out of game assets you do not own; however, compilations of specific copyrighted works can be independently copyrighted without being considered a derivative of their components[0].

That being said, the EFF's amicus brief is really good and I agree with all of it, so I'm obviously expecting it to be shot down by a court system that consistently insists on treating copyright as a federal contempt of business model tort. Remember, it took Google like a decade just to get Oracle off its back for the crime of stealing... 30 function declarations. No actual code, just names and argument types.

Anyway, do we know what Oracle was actually suing Rimini for?

[0] e.g. that one bloke who copyrighted his MTG deck to troll the Copyright Office