As all the new allied races have been based on existing ones but with some change in appearance, I started thinking of what could best fit the orcs. Mag'har Orcs, brown and uncorrupted Orcs, have been the most talked about option, but their limited difference with existing Orcs(only skin colour) might not make them interesting enough for it. Hence I arrived at Fel Orcs.
What are they?
They're Orcs that drank the blood of pit lords and became corrupted by fel magic. They can have a red, black or grey skin colour, they have spikes on their spine and/or arms, enlarged tusks and their eyes are glowing red or green. They can also be larger in size than our regular orcs.
Their spikes might stick through armour, but we already see this on Forsaken, so it might not be much of a problem.
https://i.imgur.com/iE2PLI1.jpg
https://i.imgur.com/1vmwU1t.jpg
How could they be added?
During the Legion expansion, we haven't been shy of using the enemy's power against them, Demon Hunters and Warlocks being prime examples of this.
Despite Burning Crusade lore being rather shaky, Fel Orcs seemed to have a mind of their own and worked alongside Illidan in his fight against the Burning Legion. With our defeat of the Burning Legion itself, their connection with the Burning Legion could be severed entirely.
There have also been attempts in the past to cure the Fel Orcs.
Where would they come from?
There's two major options, either we go with the Fel Orcs from Outland or we go with the ones from Alternate Draenor. Obviously the models would be the ones from Draenor as the Outland ones are hopelessly outdated, but maybe staying away from actual Alternate Draenor might be for the best.
Who would lead them?
This is a bit of a tough one, as we killed Kargath Bladefist in Outland and Kil'rogg Deadeye in Alternate Draenor. My suggestion would be a joint leadership of Jorin Deadeye and Grillok Darkeye, both of the Bleeding Hollow Clan, one is a Mag'har Orc and one a Fel Orc. The playable Fel Orcs could then largely come from that clan.
If we're going with alternate Draenor Fel Orcs, maybe Grom Hellscream can lead cured Fel Orcs that still maintain their Fel features.
Why Fel Orcs?
They're a more unique option than just skin colour, there are already good recent models of them(male and female), they have an interesting backstory and could provide for more interesting story developments. With the Alliance looking towards the Void for new recruits, the Horde could look towards Fel.
If we're going to be fighting the Alliance, why not use soldiers that were able to bring a previous Alliance to its knees and managed to kill a demi-god.
What classes?
Warrior, (dark) shaman, hunter, demon hunter(*), rogue, mage, warlock
(*)The Fel Orcs were part of the Illidari, it makes sense that if any other playable race was able to become DH, it would be Fel Orcs.