Reading some of the other posts about the fan-fiction article reminded me of an assignment from my creative writing poetry class.  We were given several confining stipulations that forced us to think outside of our normal range.  The challenging aspect of the confinement actually helped to produce much better level of writing.  Fan-fiction can cause the same reaction, because the rules are established within the original work.  Although "cheating" by changing the rules established within the original work is not unheard of, some of the best fan-fiction accepts the challenge of working with and against the established rules.  It can actually be pretty fun.