Yeah, that's an awesome idea - though obviously a lot of extra work to actually accomplish. The fiction of a fictional universe: would that be fiction squared, or fiction^fiction?

It reminds me of my brother's best friend, who is a musician and writes songs (among others) by an imaginary band called Camelot's Destruction, which is based on a dream my brother had. And the music is awesome! Check it out: http://www.myspace.com/rebeccaangel