I have a feeling they will introduce a 'neutral' race that physically differs when you go horde or alliance.
An ancient race on planet x has been taken over by the legion and is now a legion stronghold.
The corrupted natives are fighting against the rebelling natives.
They are fighting a war which no one knows when it even started.
There is heavy legion activity all across the planet.
You start neutral, in an ancient tunnel/underground system named the Archive.
You are one of the few who fled the battles and sought safety with the Librarian.
The Librarian was once the adviser/right hand of the Ruling Monarchy.
When the Legion attacked the palace of the Monarchy, he fled to safety together with other survivors.
Remaining natives started a rebellion against the ruling Legion, but a lot are being mind controlled by fel energy aka corrupted.
The Archive is under attack. You hear Sargeras whispering doom and shizzle.
The Librarian assists you into fighting against Sargeras' madness and uncovering what he wants in the Archive.
The more you progress in the Archive, the more legions appear until the Librarian sacrifices himself by collapsing/incinerating the whole complex.
You continue on the surface now.
You can't trust any of your kin, yet you are easily fooled because you are not aware of the fel energy.
you get quests by both sides, they are testing you.
The more you progress the more choices you have to make until the last quest...
The last Quest defines and picks your Faction.
Here you have to choose between saving either a corrupted or rebelling native.
One of them will be your race/faction representative.
You save the corrupted one because you feel pity and you believe in redemption. You save him by a blood pact ritual which in the process corrupts you too.
You save the rebelling one because you sense valor and you believe in hope or something. You save him by an arcane rift ritual which in the process empowers you.
You know the rituals because the librarian gave it to you and entrusted you to put it in good use.
The corrupted natives are joining the horde.
The rebelling natives are joining the alliance.
Appearance of said race:
Humanoids, not demonic, but share various traits.
Don't have hooves!
Their skin looks like avatar/Na'vi.
No hair as we know it, instead some rough/spiny stuff form beards and 'hair'.
Corrupted natives have eyes that glow (yellow to green-ish) Their face look spiny-er (black-ish/reddish) and their 'hair' vary from dark red to fel green. Have fangs.
Rebelling natives have white-blue eyes. Their faces are less spiny, their spines are white and their hair vary from white to yellow/blue-ish etc.
hidden natives have no glowing eyes, and have less to none spiny stuff forming beards and hair. they look all the same. (you at start)
When you choose your faction, character customization pops up and when finished a cinematic plays where you kinda ascend into either something dark or something colorful.
What happens next:
Sargeras has commanded to attack Azeroth via planet x. You and your chosen faction try to sabotage because you have read in the archives that titans had a link with Azeroth.
You want to learn more and escape the planet and quickly jump through the portal.
You know that meteor shower on azeroth Wrathion showed us?
You, faction members and legion soldiers are encased in fel shields and are falling to either somewhere close to orgrimmar (corrupted natives) or stormwind (rebelling natives). (+ pre-expac event)
Once you have landed, the journey was rather harsh on your kin and you all passed out. a random orc/human sees you, approaches you carefully thinking you are demon.
but for them you look weak and you look nothing like they've seen before. they take you to the 'hospital', you wake up and see your race/faction representative discuss matter with warchief/king. Your representative approaches you and says they might have found a new purpose in an alien world.
The Planet itself will be connected in a way with outlands. The planet also will include the starter zone and new levelling zones. The planet it self is not big, but it has a core and mysterious tunnels/dungeons within. The Archive is rather a wing of a whole complex (library/Science Dome.)
I expect an expansion or 2 (especially Queen Aszarah) before we fully engage legion. With legion i mean another stronghold that is of importance for sargeras. Hence that is why Sargeras won't appear physically in my suggested idea/expac.