The chance of them increasing the faction count is next to zero. It's the antithesis of intelligent multiplayer design today to splinter your playerbase, creating all kinds of issues in the process, when you can instead have one pool that can also by dynamically distributed whenever you need opposing sides in a gameplay scenario. There is a good chane that they will hold onto the 2 existing one, a tiny chance that they might reshuffle the playerbase by letting them pick sides semi-permanent and a fairly high chance that they might open up the factions in some way, just villy nilly creating more though? Nah.
Most likely scenario is just that they allow us cross faction grouping for instanced gameplay. Horde and Alliance will remain, but I no longer have to find some randos for my alliance alts to run a few m+.