I have several friends who are into anime (I myself am not a strong anime/manga fan as I was in my earlier years). At my old high school, we had a "Japanimation" club-everything Japanese, but mostly anime. These were my friends. Many of them, I included for a short time, have written fan fiction. I know one girl who continously creates "projects" for her fan fiction. I recall asking her why she does not just be original. I guess this was the wrong question to ask as I should have asked why does she not be creative and stay away from pre-created works.
Now, I am not all against fan fiction, however, if you are earnestly trying to become a *real* writer, then you must rely on your creativity and imagination. It does no good to write in pre-conceived ideas. How then can you work your literary muscles? A story by you is much memorable than a story in another person's setting.