I'd love to see the 2nd BG changing once every 2-3 weeks. It'd make sense since the Twink Cup will be featuring more than 2 BGs.
Random would completely kill the ques as it would be trying to fill individual games. You'd think the coding would be rather dynamic, but Blizz hires the best coders in the world to work on their next money maker (xpac) and not the live game.