This is an idea that I've been pondering lately. I it started with me thinking about how servers that are mostly Alliance or Horde dominated could benefit from the auction house being cross faction. On my server it can be very rare to find some materials on the auction house for the Horde side, while, from what I've observed, the probably would seem nonexistent. By crossing the auction houses together both sides would have access to more items, and those who sold items regularly would have a larger group to sell to.
But it made me wonder, what if all auction houses on a region were linked. I imagine doing so would be very hard, so this is all theoretical. The main thing that comes to my mind would be the low population servers would get the benefit of a larger variety of goods.
Just curious as to see how everyone else might theorize on the subject.