Writers all tend to have their own worlds. Though, they are considered to be original works if you don’t use any fandoms. Though, if you are a fanfiction writer you may be using a pre-existing world with that fandom’s characters. You are perfectly fine with that as well.
When you are using the fandom’s characters to rewrite part of the story, but you are keeping the initial pairing together, that is when writers alike will start to consider your work as part of the canon characterization.
Or, unless you have something very cliched, such as where the prince will get the princess type thing, then you are still in the right characterization.