Originally Posted by
Wildberry
I would really rather not burn resource-intensive, time-gated Allied Race slots on Shadowlands races.
There's very little reason for any denizen of The Afterlife™ to come to the mortal world & participate in mortal affairs. We, currently, are threatened by goings on in their realm, it's not really a two way street, even if we take a charitable view of Sylvanas as a mortal actor.
They would run the risk of being out of place almost immediately after the expansion ends. We might get a few cameos. Perhaps Venthyr Quest Givers in 10.0 will torture some named mob for information in a throwaway quest, but for them to be active participants in the story presents a challenge that Blizzard's writers just aren't capable of living up to. There's not an immediately obvious way to have them active on and integrated into Azeroth without cheapening and undermining their raison d'etre.
Additionally, even though the primary focus right now is on the Shadowlands, there are far better Allied Race candidates elsewhere. Races that are more popular, more established in lore, have better reasons for joining one of the two factions, and don't have the issue of staying power that an inhabitant from Revendreth or Ardenweald has.