That strikes me as really difficult and taking it too far out of the fanfic realm (not that I'm any expert, mind you>)
But as I see it, fanfic is grounded in a common base of a universe (Dresdenverse) of common history, experiences, laws, etc, and even superimposing the regular world of the past, there is a common base there too that we all know, or can at least Google, and can draw from.
In going into the future, the players lose that common understanding of social evolution and technological advancement. Five, ten people bringing completely different visions of the future to an RPG seems like it would muddle it the storylines horribly.
We've had some challenges on our board with people bringing characters in that really can't exist in Jim's world, and it makes the storylines difficult at times. But they're good and interesting writers, so we work with it, and we've made a non-Dresdenverse storyline area available for them as well.