Would to have a schism within each faction where when you level you get to choose between them which impacts how you level and perhaps what factions you belong to after you level up. I think it would add something to the leveling experience and create more room for storytelling within an expansion. So as an example of what I mean, you have a problem of some sort like an Old God rearing its head, then you have the various leaders for each faction(Horde/Alliance) getting together to figure out what to do and you have Vol'jin wanting to do one thing and Sylvanas wanting to do another and so he says 'this is what we are doing' to which she replies 'go to hell troll, I follow no one' and which gives you as a player the choice of who you will be following during the expansion and which quests you get in some zones. Obviously there would be some quests anyone could do in each zone but the primary story based ones would depend on whose group you were in. Same thing with the alliance except it would be slightly different.