One thing I never seem to have problems with are ideas for stories. (Quick knock on wood) Ideas can become problematic when I'm currently writing a story and they pop into my head. It's easy to become sidetracked and forget the story I'm writing to begin another. I think that was my problem the first time I sat down to write. I started about three different stories, but never really got into any of them. I blame it all on the ideas. The tricky little things start playing like a movie which makes it hard to concentrate on the story I'm working on. I've found that opening a new file and writing down everything that's come to me and tucking in the idea file quiets it down. This makes it nice when my current project ends because I open my idea file and all the documents in it. I then read through the files and pick the one that calls to me. Right now I'm completely immersed in my NaNo story. Occassionally glimpses of stories in the same world pop into my head. No concrete ideas, just little snippets that have no meaning or direction. I will probably do a brainstorming session after NaNo is done to see what I can come up with.
The biggest problem I've seen when an author uses the same world is that they build essentially the same story. This would be a huge risk with the world I've created for Fallen. Putting a new spin on things is the tricky part. It's like when a sitcom or series starts being repetitive. Sometimes it's good and sometimes it makes you want to yawn. I want to keep my work fresh which in the genre of romance is hard. It's one of the reasons I haven't gone the vampire route. Everyone and their brother has done vampires or created an "original" vampire world. I love vampire stories and the whole concept, but I can't come up with anything particularly fresh when it comes to vamps. Maybe after I've published a book or two [fingers crossed], I'll be able to revisit my friends. Right now, I'll stick the ideas in a file and lock them away until my current creation is done.
I swear my brain works in circles. I start writing about ideas and then worlds, go figure.