I've been knocking around this concept for awhile, and I thought I'd finally write it down and see what people think.
There are a lot of races among the Horde and Alliance that are allies or nearly allies, yet don't have a big enough population or class diversity to belong to a faction. So here's something I thought of that could solve it: A new neutral "race" that is made up of several races, each with one class, and with catch-all racial bonuses.
[Jack-of-all-Trades] - Passive: Increased skill in all secondary professions by +15
[Whatever It Takes!] - Passive: Increased skill in all primary professions by +10
[One-of-a-Kind] - Passive: Bonus 5% Experience and Reputation when not in a party.
[Foreign Mystery] - Active: Buff the members of your party to give them +1% to their highest stat for 1 hour.
Warrior - Ogre
Ogres, spread throughout the world, are now finding themselves at risk. A few have begun to join the Horde, while others are finding help among the Alliance. The hulking beasts aren't good for much else other than brutal strength.
Paladin - Tuskarr
Technically not Paladins, there are known Warrior-Healers among the Tuskarr, and only they have the strength and will to leave their frigid homeland to explore the world.
Hunter - Naga
As they call themselves - Tidehunters, a few have betrayed Azshara and have left the depths of their seas to find new allies.
Rogue - Hozen
Hozen Mercenary Assassins, joining whoever pays them the most.
Priest - Jinyu
Jinyu are spreading out and sharing their wisdom with the two major factions.
Shaman - Arakkoa
With dangers spreading to Outland, the Arakkoa are picking sides.
Mage - High Elf
Similar to the Blood Elf model, this one has blue eyes and paler skin, giving the horde the option to look different, as well as finally allowing a minority of the Alliance to play as a High Elf.
Warlock - Satyr
The Satyr are leaving their hiding places. While they are able to find more friends among the horde, some have managed to find old family ties to bring them into the Alliance.
Monk - (No New Race)
Druid - Furbolg
More experts of the wilds have joined Moonglade - the nearby Timbermaw Hold have sent their druids out to help aid nature.
Death Knight - Vrykul
Continuing their dispersal at the fall of the Lich King, more of his former servants are finding their place in the world.
These specific race-classes could start in "mini-zones", perhaps just being a phased village with a few quests, letting them choose their faction at about level 5, and sending them to Goldshire or Razor Hill to "Prove their worth", and at level 10 getting a special quest to meet their faction leader.
Instead of all the usual effort of creating new zones, balancing racials, classes, etc, more time could be spent making the individual race models and options.