As the comment section is closed, I thought I ought to add my two cents here.
Frankly, I sort of pity authors who don't allow fanfiction. I adore Harry Potter because I adore the fandom that has been built around the original story that I loved, which was wonderful in an of itself. However, I have lost almost every scrap of affection I have felt for Rice's novels. I have read a few and watched the movies in the past, yes, but then there was nothing more. No continuation of plot, not what-if, nothing. The fans were unable to create anything, so the fandom remained stagnant. Nothing new meant I eventually lost interest in the characters and now don't really care to read more of her work as I really don't want to be disappointed again.
The Harry Potter novels themselves were quite different. The writing may not have been the best, and it definitely seemed to decline (in my opinion) after the fourth volume. Nevertheless, I still loved it and wanted to know more. The fans had kept my interest alive in the months between releases, and I still read fanfiction to this day as there is always something new to check out. Every time I go on an archive website I reaffirm my love for a book or movie, and despite what I think of the author's writing style, I still adore the story they gave to their fans and will continue to do so.
Likewise, doujinshi keep my interest in anime that I watch, and has the same effect as fanfiction (which it essentially is, just in comic form). I probably would have lost interest in many shows while waiting for new episodes if it weren't for the fan-made stories that I have found which help to keep my interest firm. If a scanlation isn't available I amuse myself with what I can figure out from the pictures and sometimes even attempt to translate it myself (which I credit as my Japanese practice, despite my ineptitude).
My latest fan-made interests?
http://www.fanfiction.net/s/3766574/1/ (An unfinished Harry Potter fic, very interesting)
http://halkuonmusoka.livejournal.com/3800.html (Amusing Natsume dj)
http://www.youtube.com/watch?v=ckf2Ygq5r68 (Cover of "Crazy" by that new viral music guy - It's a way awesome version of a much loved song)
And now I'm off to find more! <3