Doomed to be a Fan-Fiction author?

I've been re-reading my stuff, and realized some of my best work takes place in someone else's sandbox. My Mercy story, my Phoenix story, both (In my humble opinion) really good stories with genuine feels in them.

Now, I'm not sure what this means for me going forward. Can I take something from those stories, and put them into a unique setting, or am I doomed to only be able to do good work inside someone else's universe ?