I don't see why either of them would be on Argus. They are running things on Azeroth. The combined forces of the Ally/Horde are being led by Khadgar, Velen, and Illidan on Argus.
Sylvanas will certainly interact with her sisters in the near future, but if Alleria is staying Alliance (she is) then it's likely that Vareesa will be killed (per the images of the burning Teldrasil in BFA).
Anduin will continue to be hyped by Greymane into war, then likely call on Jaina for aid (since he's pretty close with Jaina). I assume Anduin will marry the Menethil girl to solidify Lordaeron's ownership.
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