What makes a text fanficable?
Now, people write fan fiction for all sorts of things; Books, TV Shows and apparently even Fictitious Marching Bands. There's a lot of fan fiction out there for a plethora of fandoms, people and objects. There are also some things that you'd expect to be swimming in fan fiction both good and bad which seem to be largely ignored. I'm thinking of a series of books or a tv show which is obviously popular, but not heavily written for. Yes, you will find that there are some stories lurking in the murky depths, because I'm pretty sure people have written 'fan' fiction about amoebas, but not the hundreds upon thousands of fan-written words that other texts influence.
What inspires the creative juices to start playing with the characters, settings and nuances of another text? What are the elements contained in those fannish items which makes you go "yes! I want to write stories about that!" What will encourage you to want to read fan fiction of a particular item?
On the flip side, what might make you not want to write for a particular text that you enjoy? What might make you not want to read the fan fiction of a particular text you enjoy? Are there certain elements contained within texts which deter the writing or reading bug?