If you don't want to worry about copyright issues, then yes, use only material you have made. But that's not what's required. People are allowed to use some aspects of other people's copyrighted material legally. How much, and in what way... that gets complicated.
A lot of us want to build in what other people have made - want to show it in a new way, or have a different perspective on it, or use existing characters with different game mechanics.
I tend to think this is transformative use, not derivative - but I am not a video game developer and not a lawyer; it's not my opinion that matters.
I can say that the most common reaction to a big company deciding something is derivative/infringing, is to demand it get removed; there is almost never any other penalty. (Going to court takes money. Even if they would win the court case - there's no point in suing a small indie developer; they won't get that money back because most indie devs don't have it to give.)