The final allied races for BfA will be revealed just after the book Before the Storm comes out. The Darkfallen will be another Allied Race joining the Horde, while the Redeemed will be joining the Alliance.
In the Darkfallen recruitment scenario, Koltira Deathweaver returns from a recon mission in Northrend to report of a civil war between Bolvar's undead empire and the remaining Darkfallen that wish to retain their willpower. You are sent along with Koltira to Icecrown on a covert mission to meet Blood Prince Dreven and steal the newly completed Malykriss: The Vile Hold from Bolvar's control. The scenario involves political intrigue amongst the Darkfallen who pretend to serve Bolvar with the ultimate goal of creating a diversion just when Malykriss reaches completion and escaping Northrend in it with the remaining Darkfallen. Apparently they are dissatisfied with Bolvar's refusal to let them scour the lands to the south for living subjects to satisfy their blood thirst, and have resorted to drastic measures for sustenance making them appear extermely malnourished. Sylvanas agrees to let them satisfy their blood thirst in her war against the Alliance so they agree to join the Horde. Once Malykriss is liberated from Bolvar's control, it is relocated to float off the coast of Darkshore near the remains of Teldrassil where the Darkfallen help the Horde clean up the remaining Night Elves who have decided to stay and fight for their lands instead of fleeing to Stormwind. Malykriss: The Vile Hold is the racial capital for the Darkfallen. Aesthetically, it is a fortified Black Citadel floating off the coast of Darkshore with a view of the remains of Teldrassil. The interior is designed in a similar fashion to the Crimson Hall in Icecrown Citadel and is populated with chained Night Elves and Darkfallen Handmaidens (Night Elves who have been killed by overdraining and became Darkfallen themselves) whom are being kept for feeding purposes for their Darkfallen overlords.
The Redeemed are the Forsaken who were killed by Sylvanas when they attempted to rejoin their living siblings in the Alliance in the events of the novel Before the Storm. They are led by a ressurected Calia Menethil. The recruitment scenario for the Redeemed involves going to Hillsbrad Foothills/Arathi Highlands to resurrect those who were slain in their attempts to rejoin the Alliance, while defending against guerilla strikes against your forces by Forsaken remaining in the area. The Redeemed base of operations will be Uther's tomb where they are gathering to resurrect the fallen hero with the ultimate goal of securing the Capital City of Lordaeron with his help at some point in the future.
Both Darkfallen and Redeemed appear to be based on the datamined "thinhuman" models which are themselves Forsaken models without the hunchback. The Darkfallen have a sinister Elven/Vampire appearance while the Redeemed appear similar to the Forsaken but more upright, without exposed bones, and white eyes instead of yellow.