Interesting take on it - about fanfiction being a hindrance for you because of the established characters.
The funny thing is AUs give way more 'freedom' - and I was more prone to writing those as it made me just take the characters' main personalities and dip them into my own world.
That in turn gave me the ability to create my own unique characters, flesh them out, and get them to interact with the 'established' characters...if that makes sense.
(oddly enough though, I was already stories when I was a kid before fanfiction gave me the avenue to branch into originals)
In addition, though I've heard of those anime, never could get into them for whatever reason. *lol*