Not sure if this has been brought up before, but wouldn't it be pretty amazing with a DoTA-style battleground in WoW?
For those of you not familiar with Defence of the Ancients, or MOBA as the genre is now called (League of Legends, etc.), you have 2 factions - each with a corner of its own for bases, shops and spawn points. Then you have lanes running diagonally across the map, through the wilderness, between the two bases. There are also smaller paths through the wild, short cuts if you will, where you can find monsters.
NPC units race down the lanes towards the enemy bases, and it is up the players to defeat those units as well as the players from the opposing faction. You level up, have skills, purchase gear, etc. Once the enemy base is destroyed, you win the game. See the image below for a very basic layout. The Red and Blue squares are opposing bases, and the yellowish squares are contested regions in the middle where you fight. The grey are lanes.
So imagine if we could have that in battleground style, but that you could also get loot and experience from all the NPCs, monsters and bosses you kill during play? The main difficulty would obviously be PvP and keeping the other players in check, while also trying to destroy their base. But I think being able to kill monsters and bosses would be a nice boon.
Certain rules would have to be put in place to make sure none of the two sides cooperate to milk the battleground for the monsters.