You can see big and epic PVP in EvE Online for example. For regular member it looks like "orbit/follow anchor and shoot/heal broadcasted targets". There's no skill involved, except from few people. And it's all about movement and positioning. You basically want to be on your optimal range, your enemy in bubbles so they can't run away or rewarp on their optimal range, and your healers outside of enemy's optimal range. In most cases if enemy blob targeted you - you'll die, logistics can't outheal alpha strike. So in WoW it will be big blob of hunters and druids against other blob of hunters and druids. Those classes will be best because they have a lot of instacasts and big enough alpha strike. And it will be about pure numbers - there's no difference in range between different ranged classes. No fun at all. Or they have to rework entire game from scratch.
40 vs. 40 is pretty small for "army vs. army" scale.