An Alterac Valley style zone COULD be fun, but it would need to be on a different set of rules from the traditional warzones, at least so far as queues are concerned. When AV was first released, games didn't end very often on my server (Alliance sucked and Horde rarely had more than half a full team). I'd rather not be "stuck" in an unending warzone, I'd want the freedom to jump in and out and see what I can do, see if we can't win it.
Alterac Valley games would last 3-4 days on my server. That wasn't a server queue issue though. That was because it was so hard to actually make progress to win. There was no way to burn towers to remove points from some pool so the game eventually ended.
Assuming you make the rewards for winning this huge battle style Warzone good enough, you wouldn't need cross-server.