I made a whole post about it here:
https://us.forums.blizzard.com/en/wow/t/sanlayndarkfallen-coagulated-arcustomization-idea-megathread/18803/7485
If that happens, Kael’thas would be back on Azeroth. Meanwhile, the San’layn would find themselves with no Lich King and possible no Jailor controlling them. At the same time their Prince returns from the Shadowlands as a cool supervampire, and nothing better to do but provide guidance and leadership for his equally undead people.
And then, in need of allies, it seems the logical conclusion for the Undead Blood Elves to join the faction with Undead and Blood Elves.
Don’t care. Want Black Drakonids in the Horde and Blue Drakonids in the Alliance.