Some horrendously bad reasoning here; never going to happen, ever. Alliance vs. Horde is literally the essence of warcraft, it's called warcraft for a reason. That'd be like having the terrans, protoss and zerg merge in starcraft and forming a unified faction to fight the xel'naga, instead of just, yknow, working together temporarily.
Merging the factions would destroy the whole dynamic of having factions to begin with, only to have an infinitely more forced light vs void? Lore wise you really think people would side with the old gods and the void lords just because the naaru arent perfectly good beings? And if they did, wouldnt that just be blatantly good vs bad, the very thing they've tried hard to avoid doing with the Alliance and the Horde by making them both shades of grey? or we're all part of a merged faction and yet we still kill each other in bgs for some reason?
the entire concept of eliminating alliance vs horde is just stupid wishful thinking, and the only reason i could ever see it happening, ever, is if the population dwindles and they have to do it for the sake of population control. Even then, it would be more a matter of just making all servers PVE and letting cross factions communicate and group/guild, it wouldnt be removing the factions. The core of the game is AvH, it's two distinct factions that give you two perspectives on the world and on events that take place, eliminating that would make the game strictly worse. Especially given the situation with the twin emo dykes of warcraft(sylvanas and jaina), and genn out for blood, a peaceful merge, even lore-wise, is several expansions off at best.