It would be a stretch, but I think the Faceless would be a very fitting race if there was an Emerald Dream expansion pack for several reasons:
1. They are related to the Old Gods.
2. The Old Gods are related to the Nightmare (their creation), which means the Faceless are related to the Emerald Dream
3. They have very limited lore, so it is possible to fit them into an Emerald Dream EP.
My idea for their lore would be:
Once the agents of the Old Gods, the Faceless followed their supreme masters, corrupting the Emerald Dream through the Nightmare as their mission. With their deep affinity to nature, the Faceless were able to enter the Emerald Dream and from there, pursue the Old Gods' bidding. However, when the Titans defeated and locked away the Old Gods deep beneath the world, the vile, bloodthirsty spirit, Hakkar, took control of the Faceless. A strong hatred for the tyrant, a large group of Faceless broke away from the others and renamed themselves the Zealots, fanatically searching for traces of their Old Gods. When they learned of Hakkar's traitorous act of calling the attention of the Titans of Azeroth, the Zealots began going against the will of Hakkar, which was to continue spreading the Nightmare in the Emerald Dream. They began fighting against the Nightmare, alongside the other Druids of Azeroth. Now allying with the Horde, the Zealots try desperately to find the Old Gods and defeat the Nightmare.
Or a longer version:
Creatures of the Old Gods, the Faceless were led by them, but were also under the control of the vile and dispicable spirit, Hakkar the Soulflayer. These Faceless were agents of the Old Gods whose mission was to taint the Emerald Dream and create a mobile area of corruption known as the Nightmare. Having a magical affinity for nature from their birth, the Faceless began dreaming of the Emerald Dream, and from there, began corrupting it. The Old Gods' plan was a success and the Faceless continued to do their work. However, when the Titans locked the Old Gods deep beneath the world, Hakkar took complete control over the Faceless, to many's dismay. Those who followed Hakkar continued to taint the Emerald Dream as he wished, coming with him to his temple in the Swamp of Sorrows to do his bidding. The others hated this bloodthirsty tyrant, believing that he wanted to overthrow the Old Gods anyways. They seperated from the other Faceless, renamed themselves the Zealots, and began their search to find their locked away masters. In the process, they learned that Hakkar was a traitor who actually brought the attention of the Titans to Azeroth. This was enough for them to go against Hakkar's tainting of the Emerald Dream. Even though spreading the Nightmare was the Old Gods' mission for them, they could not follow the works of Hakkar. The Zealots began to strive to fight against the Nightmare and hold it back from their corruption-spreading brethren. Allying with the mighty Horde, who they encountered on their search for the Old Gods, the Zealots create their home north of the Swamp of Sorrows, constantly battling it out with the Faceless of Hakkar in the wakes of the Emerald Dream. Although the Horde do not know the Zealots' connection to the Old Gods, they find the Zealots useful allies, as they are able to enter the Emerald Dream and fight back the Nightmare alongside the Tauren.
*Notice how Zealots mean a fanatical follower, so it would be perfect for a Faceless searching for the Old Gods.
My chosen classes for them would be as followed:
1. Druid (Obviously. If there was going to be an Emerald Dream EP, there would need to be a druidic class. Since the Faceless are able to enter the Dream and spread the Nightmare, the Zealots can enter the Dream and fight against it)
2. Hunter (Their relationship to nature should also relate to a relationship with animals)
3. Priest (Just to balance out the current classes, but it would still be fitting)
4. Rogue (I can imagine them being Rogues for some reason)
5. Warrior (No explanation neccesary)
Faction: Horde
Mount: Since their appearance looks very alien-Protoss like, I was imagining them mounting a giant floating jellyfish. I call them Siphongels.
Language: Since it is Unknown what race speaks Abyssal, I would give it to the Zealots since they seem like an abyssmal race.
Starting Area: If they were to fight against their brethren in the Swamp of Sorrows, I would give them a starting point north of it.
Would this idea be a stretch or would it make sense in a way? Comments would be nice. If positive feedback is given, I will continue on with the starting zones and yeah.