i organized something similar to this on ED a few years back
http://www.wow-one.com/forum/topic/4619 ... Bashenvale
we just chose areas with interesting terrain and had players meet at those coordinates to fight. usually this would result in an initial fight that was someone staged/set up and then the battle would progress for 30-45 minutes until most players were dead or scattered around the area.
here are a few vids from it:
The First Clash - http://www.twitch.tv/dragmyre/c/2246243
Battle for the Island - http://www.twitch.tv/dragmyre/c/2246259
Battle for Raynewood Tower - http://www.twitch.tv/dragmyre/c/2246410
anyway, my main word of advice would be that if you want it to be successful you need a very clear leader/organizer on each faction, and they need to be in close contact with each other throughout the whole thing. also, having a global chat channel set up for the event on each faction is a good idea because it allows all the players participating to communicate even if they are in different groups/raids
anyway, good luck