[@Thundercrash]
Neither.

Go look at some other books, movies, etc. Find some sorta-similar ideas or ideas that you like.

Mix the original idea and everything around with the new ingredients to become something new.

"Originality is hiding your sources."
None of my ideas, however 'original', are original. None of the names, none of the appearances, nothing. And, I think, nothing ever really is 'original.' It's all inspired or from somewhere.

OR!
You can say, hey, this roleplay/story is an AU of this really long-standing book series.