I don't think it's ever going to be something that happens. Or if it is, it's going to be very limited. Or maybe when Blizzard announces that WoW's dev team is going to be cut, à la Starcraft 2 and Heroes of the Storm. And merge the factions to reduce the amount of work it takes to make separate quests for both factions.
The Alliance and Horde split is one of the core identities of World of Warcraft. While there have certainly been moments where banding together makes sense, like with the Burning Legion banging Azeroth, there have also been lore explanations where being separate factions is more desirable.
Anyone remember this from Mists of Pandaria? (relevant part starts at 1:30)