Players have been asking for a new battleground type for a while, and there are plenty of great ideas out there. So what's holding Blizzard back from creating a new one? Nethaera has said in the past that Blizzard is worried about queue times-- more BG queues to join means longer waits for players, and on some servers, the waits are already long as is.
So how can we fix this? Relmstein has a few really good ideas-- basically, he says that any new battlegrounds shouldn't replace the ones we have, but rather join the same queues. Instead of choosing from all the battlegrounds, AV and the new battleground (Relmstein pulls the idea of Alterac Ruins out of the air), would share a queue. That way, players wouldn't have to wait as long, and the new battleground would get just as much attention as the old battlegrounds.
Halo 2 (which I've been playing lately in expectation of Halo 3) has a system like that-- instead of joining a map, you join a "playlist," which has a number of maps and gametypes in it. That way, you only have to choose from a short list, but still get to experience all kinds of possibilities. And in this way, Blizzard could make variants on the fields we're playing now-- a new CTF map (like WSG) or a new node map (like AB) sounds like a lot of fun to me.
Of course the drawback to that is that if Blizzard comes up with an entirely new BG (like the rumored Scryer vs. Aldor BG), there's no real reason to queue it with AV, except for this problem of queues. Then again, for most players, pairing different battlegrounds isn't a problem, as long as it gets them in the game.