@Faction War
I've often said they need to create a neutral faction. A group of people who are fed up with the faction war exchange who wish to be part of neither the Alliance nor the Horde. But, the logistics of that would be troublesome.

Neutral races such as the Pandaren are probably what we are going to see more of in future (So said the mighty Ghostcrawler before he left, as did Metzen and Pardo), but if there were an option to stay neutral as a supported means of playing the game, that would be pretty cool. Or if perhaps at some point after seeing X levels worth of content one could chose to be Neutral.
IE-Stabby the Tauren Warrior starts out at level 1. After a few brief moments of wishing to be one of the awesome Tauren Rogues, Stabby sets out, does quests, does dungeons, and levels up to 80. After the Wrathgate event, Stabby busts out his table, tableflips, and says "I'm out. Peace yo!" and undergoes some other event or chain of events (combo of solo scenarios and open world quests) and discovers the Neutral town of Neutralopolis. Filled with others who conscienciously object/abstain from the Horde/Alliance struggle, and only work towards the betterment and protection of Azeroth as a whole.

The Problems:
-Would it be a paid service option? Why? Why not?
-3rd Faction could potentially destabilize the raiding community and the PvP community. Would they even be allowed to PvP in Battlegrounds or Arenas?
-How would it affect servers who already have a major faction imbalance?
-How would grouping work? Could you group with Alliance/Horde or just other neutrals? How would it affect the queue times?
-Would racials need to be rebalanced? Again?
-Would certain races ONLY be Neutral? Would some existing races be excluded from going Neutral? (IE-After Garrosh, maybe Orcs aren't trusted enough to be Neutral for a while)
-Would neutral open up new class options to certain races? (IE-Tauren Rogues are too silly for the Horde, but the Neutrals don't judge)
-Holiday events?
-Story reason for this faction to form are numerous, but audiences tend to need a single unification motive/event. If they've been around for a while but in secret, they need to be slotted in somewhere without stepping on the Lore. If they are new, they need to write an event to announce it story wise, and they would need to involve them in other story events as time goes on. Something like the Dominance Offensive would instead need 3 camps instead of 2, NPC's have to have their aggro AI adapted to include the 3rd faction. Lots of things that sound easy on paper, but make for a mountain of work when added up, especially if it requires engine-based changes.

I think it would be a fun idea, but a nightmare for the developers. Probably too big a change to impliment now, and after Cataclysm they are highly cautious of large scale changes. And as fun as it sounds, it might not actually work out to be all that fun or interesting as we think.