Fan fic, or writing fiction based on existing worlds and characters, isn't new - what is new is that it can turn into opportunity.
The Wall Street Journal covered this over a year ago (hey, I had a baby, I can be late to the discussion), focusing on Kindle Worlds and the opportunity it created for fans to legally publish fan fic (because, you know, otherwise it is ILLEGAL - do not publish/post without understanding this!). The more traditional non-profit approach to fan fic survived low on the radar of copyright infringement primarily because it didn't threaten sales of the real books. KW is done through license deals with publishers; authors can make money, but so do the publishers.
Pretty cool. I guess. As an agent whose client was approached by KW to write for one of their worlds....eh.
So here's the thing. Fan fic can be great. But not if it's going to take away from your own career.
What worries me about this new form of fan fic is that it could lead an author too far down the path of writing for someone else. This wasn't something to really worry about before; after all, fan fic could also lead to bigger and greater things like Fifty Shades of Grey, wherein the end new book is original enough it doesn't appear to be a derivative, and bing bang boom a new bestseller can be born.
But because authors are now free to publish without having to worry about masking the elements that were created by someone else, will those same authors, who might have created the next Fifty Shades of Grey, simply...not?
Or will an author so excited to be approached by or interested in KW take up the challenge...and spend time that otherwise might have gone to their own work doing it?
It's been around long enough that I can't really say it's much of a threat. But...something, I think, to keep in mind, the advice of the day on career planning, if you will - for any time spent on fan fic.