Sira Moonwarden.
Probably one of my favorite examples of night elven culture integrating into the horde through the forsaken, And if blizzard ever gets back into the faction conflict drama, I would love to see the darkwardens become prominent on the horde side as their own independent arbiters and vengeance seekers.
The scourge and the scarlet crusade:
After we have effectively learned that they are in their warring kingdoms arc post shadowlands, I would have loved to see a revival of this conflict that doesn’t involve in the complete and total lobotomization of the crusade as blizzard is desperate to write as well as an expansion of the lordaeron conflict in general.
The Drust:
The most disgustingly fumbled faction ever handled by blizzard. An opportunity to implement the true blue cruelty of celtic druidism through wicker man sacrifices and dark magic in contrast to the emerald flavour of druidism, in the eternal cycle of spring and summer versus autumn and winter in a pseudo fae court fashion.
They really should have been a lot more prominent than they were and a sort of third faction dynamic against the nightmare and the dream dedicated solely upon the cycle of nature and inevitability.
EDIT: We also still basically know nothing about the wardens. What even is an avatar of vengeance? What are shadow orbs and why do they come in different variety?