As a writer, I've had the chance to dabble in so many different varieties of writing. One of the first I started with was fanfiction and fandom based roleplay writing. I have always been a reader, and there were several times when I would read the story or stories if it was a series, and be upset with the ending the author gave.
Granted, as the author, they have the right to end their own story any way they chose. As a reader, I would always picture different endings or want to change interactions with characters.
That is the entire point of fanfiction. At least, for those of us who only write these things for fun. That is why its called fanfiction. It is fiction written by the fans, and there are a lot of authors out there that encourage it.
Harry Potter is one of the most massive fandoms people write within. If you've ever read a book in a Kindle Universe, they are fanfictions in a way. They are a series of authors writing in someone's playground using their characters or original characters with associations to the canon.
When I started writing it was clear I was green. I could barely write more than two hundred words in one setting, and it was an AU Post-Potter Harry Potter roleplaying site, along with fanfiction that helped me build myself as a writer creatively.
I learned that I could put myself out there and that not everyone was going to enjoy what I wrote. Just like there were going to be people in life that do not like me, there will be people that do not like my writing. However, not writing my ideas down I felt was worse than trying to get my ideas out there. I still write fanfiction, but I've grown with creating my original plans and putting them to paper.
While there is a significant community that supports fanfiction writers, mainly the smaller fandoms, there are those out there that make it hard for others. I've been writing for many years, and I've seen quite a lot. One of the most substantial things that we see happen, and this happens with people's actual books in Kindle, is that others will steal the entire story, only change the title and repost it.
We all know this is wrong; this is plagiarism. With original ideas in books, it's easier to get something done, but since the characters in a fanfiction may only contain one or two original characters, it is harder to get plagiarized stories removed.
No matter what you write, get out there and give a fanfiction a try. You do not even have to share your work with anyone. Sometimes we need the closure of ending the show the way we felt in our heart it should have ended.
Like not killing Professor Snape off, or taking away the death of the main character who worked so hard to protect everyone through three books. Let your creative juices flow and you'll find that you can write so many more things when you venture into that realm.