I'm sure most of us have heard of fanfiction. And for those who are about to google it, lt me explain. Fanfiction is sort of like fanart or fan letters in a sense. For example: Harry Potter has a lot of fans. Some might write their own stories about their favorite characters.
Fanfiction is a great way to express your ideas. Sure, you may be using the characters and places that were created by someone else, but you are adding to it. Each person may read a book and have a different take on it. This is about each fan's interests. One fan might see a romantic connection between characters which may not have been addressed in the book, movie, poem, etc.
It is using our imaginations and a great writing exercise! I believe fanfiction to be a great way of learning how to write in an interesting manner. Some may complain that you don't get to practice creating new worlds or characters. False. I created several new characters in an X-Men fanfic I wrote once. You can do anything. Make your ideas come to life using characters that people are very familiar with!
• 
• 
• 
• 
• 
Or you can just write an original fiction. Especially because you apparently have created several new characters in your fanfic.
This is true! But a fanfiction give you those supporting characters! I think its a good exercise to do every now and again before working on your own original work, you know? n_n
I say go for it if you like, but some people carry such things way to far and get obsessed.
Fanfiction is a good way to start writing, I did it myself for quite a while. =) It's very fun to use as a springboard into creating your own characters (OCs) or just continue where the canon story left off. The only problem I can see with fanfiction is your view of the characters' personalities can get distorted (especially if you read a LOT of fanfiction). And, of course, there's the prevalence of homosexual fanfiction now, which is kind of why I've moved away from it. Anyhow, you're right in saying it's a great way to use imagination~ I've read some really interesting plots in fanfiction. =D