Individual Instances

For a change of pace, perhaps Blizzard could make a high level instance, where you'd need to be geared, that was designed to be soloed, in that it could only be soloed.  Each player class would have a different experience in the instance, but everyone would play alone. 
Perhaps it could be proving grounds, where an individual has to prove their worth to earn an armor set, or items.  There could be multiple wings to the instance, that could all be done separately.  The instance could be a portal to another planet, to the Emerald Dream, to another dimension, etc.  It would probably be run by a group, like how the Argent Crusade ran the Trial of the Crusader. 
Basically it would be interesting to push solo play at the high level and good gear range, so players can see what they can do by themselves.  The bosses could be elites, but with strategies designed for individuals, and the mobs could be nearly overwhelming. 
Perhaps the gear could be an optional, in between Tier level, or gear to fill out the gaps in a Tier set, or even a Tier set. 

A Lore Thought

I predict that players will have to fight powerful Avatars of the Old Gods that they've killed.  The Gods' power before was in corruption and influence, but they are free from their bodies and can now assume forms which are more powerful.  I'd bet that. 

I'd also say, though with less certainty, but more hope, that the Old Gods, or their Avatars, will be banished into a portal, or to a place away from Azeroth, rather than directly defeated. 

Rewards based Races

Instead of having a new race available to anyone, why not have races that high levels could reroll as, as with Death Knights as a Hero class, have a race with each faction that newbies can't be, but experienced players could earn the right to be.  Perhaps a player would have to do quests to convince the race to be a part of the faction, with quests like escorting low levels to Beginner areas, and making a certain NPC a strong leader, and building up a city for them. 

Horde

Stonemaul Ogre

Possible Racial Traits

Enrage – temporary buff that increases size and damage dealt for 10-15 seconds

Strong - +5% to Strength

Two Headed – increases Stealth Detection

Imposing – Cast Fear effects last 10% longer

Possible Classes

Warrior

Warlock

Mage

Shaman

Hunter

+Death Knight

Mount – Mesonychid – a massive hooved wolf-type animal

Alliance

Furbolg

Possible Racial Traits

Bear Bite – does a quick bite attack

Leatherworker - +10 to Leatherworking

Insulating Fur – Fire and Ice resistant, and lessens bleed effects

Possible Classes

Shaman

Warrior

Hunter

Priest

Rogue

+Death Knight

Mount – Cave Ursine – Large, bear-like creature.

Also, perhaps instead of a specific tribe of Ogre or Furbolg, perhaps different tribes could be converted, through quests that kill the hostile leaders, and win over the hearts of the that tribe race, and convince them to join. 

Army Battleground

Why not have a battleground where players have troops to control to fight each other with. Players start with 5 troop members under their control, as Sergeants. These battles would be 20 V 20, so 100 troops and 20 players fighting each other. Each play, each Sergeant would be given an objective, like Protect a place, or NPC, attack a place or NPC, capture an item, free prisoners, kill as many enemies as possible, etc. A player gets two sorts of points. Order points they get for successful completion of their goals, Victory points they get for winning battles. These points would add up to allow the player to advance in rank. They could advance to Lieutenant and then control 10 troops, and fight with and against other lieutenants. Then Captains, which control 25 troops; Colonels, which control 50 troops; and finally Generals, who control 100 troops. In addition to using points to gain rank, a player would try to earn general armor, which would make them able to withstand large group assaults. All troops (NPCs) would be the same, basic fighters. It should also be downplayable, so that someone who has earned the rank of general could play and fight in Sergeant battlegrounds, as a Sergeant, if that's what they enjoy.

In addition to fighting and earning your own Orders, perhaps there could be extra orders, like do it quickly, or help others after you've done yours, to earn another order point. 

This battle ground would be different than others, in that there would be no spirit healer.  Though, I suppose it could be worked out so that you could respawn, with troops.  That just seems too complicated.

The troops would follow the player around, and the player would have a couple commands he or she could give the troops. They could give individual orders, and whole group orders. Orders could be follow, stay, defend me, defend place, divide into groups, ignore attackers, and attack.

In addition, there could be multiple battlegrounds, or players could earn the right to different battle grounds, which could be just a little different, which different maps and goals, to very different. Very different could be to have different classes in troops units, as in Rogue Scouts, Battlemages, Archers, Healers, Tanks (as in mechanical tanks), flying troops, cavalry, and sappers (engineers). There could also be a battleground where the player could, through times and battles, earn NPC lieutenants, captains, and colonels to fight which them. And these NPCs could have different classes, Shaman, Paladin, Death Knight, Warlock, Druid, and the rest.

Victory Points, along with Order Points would rank players up.  Say 50 Victory and 100 Order points earned would advance a player from Sergeant to Lieutenant.  100 Victory and 200 Order points to Captain, 200 Victory and 400 order points to Colonel, and 400 Victory and 800 Order points to General.  Perhaps to get additional units, a player would have to be a general, and then every 200 Victory points they could try a new 5 man troop type.  It's a thought. 

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.

New Mage Talent Abilities

Blink Other – Arcane – Can blink target player away from mage
Heat Metal - Fire - Either a spell that heats targets armor and weapon, degrading them and causing them to have a stackable fire damage debuff, or an added affect to fire spells

Dueling League

This is a One on One PVP arena style match. You face another player, Horde or Alliance doesn't matter, that's the same level as you, exactly. Except inside the instance of the dueling league, the player wears dueling clothes, automatically put on when a duel starts. The dueling gear is the same for one class, at each level. Cloth robes for Mages, Priests, and Warlocks, with no stats increases. No trinkets, no health pots, just player versus player, and gear doesn't matter. Rogues and druids would get leather, mail for shamans and hunters, plate for Paladins, Warriors, and Death Knights. Plate would give 4X the armor protection of cloth at the same level, but no more than the basic difference between cloth and plate armor. You'd get to choose a basic weapon for each class, which would be the same on each level, mages could choose a staff, dagger, wand, etc, but it would be the same. Hunters get choices of Crossbow, Bow, Gun, and get automatic bullets/arrows, but all do the same damage. Buffs from other players get removed, along with effects from potions, food, etc, upon entering a duel.
Certain class combinations are pretty unfair, like a Rogue vs. a Paladin.  No matter how well the Rogue starts, the Paladin could just bubble and heal, or use Lay On Hands.  Perhaps these could be disabled for these player combination duels, if it proves to be a problem.

Basically these duels would be about the skill of the player, not at all the gear of the player. Though some class combinations are unfair, so perhaps there could be trinkets or special health pots for dueling that a player could use to make one on ones fair between certain classes. This could be rated, and have elimination tournaments, so the best duelers end up facing each other. They could grant out of duel rewards so people want to do it more. Good off hand or trinkets for people of high rank, or high win percentage.