I'm almost 100% convinced we'll see history from WoW repeat itself in Aion: Elyos (Alliance) will initially outnumber Asmodians (Horde) but Asmodians will dominate PVP whenever they don't get bulldozered by superior numbers. Eventually people on the Elyos side will grow tired of always losing, so they start rolling new Asmodian characters, which is when the populations will start to slowly balance out.
I'm not sure I'd bet a whole lot of money on this prediction, but I'd be tempted to.
