Writing for fun, but no profit

I have discovered, through my oldest niece, the world of fan fiction. For those of you not familiar with it, it's a forum for people who like a given television show or movie to write their particular story ideas. People write Harry Potter stories, "alternate universe" stories, where things occur in a different way than the original author wrote it.
As you know, I'm a Kim Possible fan. When I read some of the stories that are in there, I just about went ape. Some of these people are really talented writers. A couple of stories really appealed to me, so I reviewed them. They also gave me a launching point for my own story.
I'm not just sure where this thing is going to go. I've started writing a thousand stories, and they all wound up in the recycle bin on the computer. They just seemed too lame. This time, using a cartoon show as a jumping off point, I don't have to create something from scratch. I just have to put the characters into a situation, and let them react normally. There in lies the beauty of fan fiction.
On the other hand, I feel like I have to be true to the characters as they were created. Many authors in fan fiction feel at home taking liberties with the behaviour of the characters, or the basic personality. I think I have to approach it from the point of view as if I were caring and feeding somebody else's pets. I'm responsible for what happens to them when the owner isn't looking.
Maybe there could be a manual for the care and maintenance of another person's intellectual creations. Of course, that would take away the fun "what if" nature of the medium.
Anyway, stay tuned, and I'll let you know how it turns out.