New Underground races

Horde

Teuthid Faceless

This faction is made up of Faceless which have gained free will, from the deaths of their Old God masters/creators/parts. They have formed a protective city because other faceless and minions of Old Gods attack them and wish to add them as their servants and slaves. The Forsaken would advocate for them. They could also be made up of a cast of smaller faceless that were the slaves and servants of the larger faceless.

Possible Racial Traits

Touch of Shadow – All attacks have 1% chance of doing an additional 2% Shadow damage

Left Handedness – Melee attacks are 15% less likely to be dodged, parried, or blocked

Sticky Skin – Lessens chance of disarms, and disarm effects don't last as long

Free Mind – Resistant to mind controls, or other mind effecting spells and effects

Streamlined – Faster swimming speed

Possible Classes

Warlock

Mage

Rogue

Warrior

*Mentalist

Mounts – Centipar – Large centipede-like animal

Deeppool Centerpool, or something. The main city would have underwater throughways, with some NPCs in pools. The guards could be warlocks instead of warriors, and they could have stealthed Rogues patrolling the exterior of the city. New city could have an instance in it for low levels to fight agents of an Old God.

Alliance
Aqir
The Insect people that were the beginnings for the Nerubians and Qiraji, but these are uncorrupted and underground.

Possible Racial Traits
Flit - Flaps wings a couple times to get character out of reach for several seconds
Winged Flight - At level 60 they learn to fly.
Carapace - They receive +5% to armor rating
Spider Climb - Can move up vertical surfaces for 10 secs. 
Sting - does X nature damage, based on level.

Possible Classes
Warrior
Hunter
Priest
Shaman
Mage
Rogue
*Mentalist
*Spiritualist

Main City - Azj'Aqir, Hivelike

Mount - Spider

In the event that Blizzard might go with a species not in lore already. 

Alliance

Agaricon

This is a race of fungus people, like the sporelings but bigger and more to them.

Possible Racial Traits

Spore – Blinds energy for 3 seconds, has cooldown

Natural Growth – Healing effects and natural healing increased by 1% when in contact with water

Unarmed Combat +5

Eat Herbs – Can eat picked Herbs directly to gain health, and some effects

Possible Classes

Shaman

Hunter

Priest

Warrior

*Spiritualist

Main City – Fungal Dome – Maybe have shamans as guards around city. A large dome, with lots of plants and glowing fungus. Lower level instance over city with corrupted plant elementals and other enemies.

Mount – Fungasaur – a fungus beast they grow and ride.