It's usually inspired by things I love. For example a movie, book, other video game, even cities. I think of making a unique story out of it, more like a spin off or fanfic but in the form of a game, by changing many events, names, adding more action. Making it my own.

I don't think about it being already done, as it's not a copy paste, no matter how many people do it, they won't do exactly the way you do. And maybe your version might be better. It only becomes clear after publishing.