^ A one-shot, is a one chapter story that's fairly short. It can be quite a few pages long or not that big at all. Basically it's a short story with one chapter, nothing too big.
In answer to the topic, it's entirely up to you whether or not you want to write a fanfic, which it looks like you are. You also have to be passionate about it. Like mentioned before, if you want to write a story, it has to be your decision, not just to please someone, but to please yourself.