Everyone here likes a good fanfic, and if your starting post sounds good I think that a lot of people will contribute.
But maybe you have looked around the forums a bit, so my next words might be not necessary at all: Many of the people here take the game universe pretty seriously and want to keep with its given timeline, technology and culture.
If you're starting a cooperative fanfic, that's quite some coordination task, because there is a pretty low endurance threshold for siliness around here. Things like genre crossovers, fantasy technology, or changing existing events / biographies will not be very well received.
My advice: Post an outline of your story, hand out character descriptions to your participants, and/or pull up some rules describing the amount of freedom you will accept.