With my idea any player can create a new "Fallen" character and play alongside Sylvanas for an entire expansion. This way players will experience being "the baddies", have an evil outlook towards N'zoths goals, and serve the future Undead Queen.
Players don't have to leave their factions, instead they create new characters, including the new class Dark Rangers within that 3rd faction.
Just like Mists of Pandaria though, towards the end the 3rd faction characters choose a side to join, leading to a whole bunch more diversity in races for each faction.