If my recent theory based on Xal’atath’s statements in the Ansurek Raid is accurate there is another possibility for Anduin’s resemblance to Arthas:
It’s actually Arthas forced to dream he is Anduin while being drowned by the 1st Harvester of Wrath(posing as Tyrande) with Sylvanas taking on the role of Jaina Proudmoore to watch over him and Jaina herself(after either getting killed rushing to fight Xal’atath in Dalaran or getting killed by Talanji while no one was looking) taking on the role of Alleria.
The first Harvester of Wrath of course will be informed by the Winter Queen(who was playing the role of Vyranoth while guiding us and attempting to aid Fyrakk-Iridikron’s redemption) that she needs the Mortals for Rebirth to stop Xal’atath only for her to vehemently refuse only to get frozen exposing her true form as she is thrown into the water and drowned by the Stonewright(who uses the Model of “Xal’atath”) who is elated to get the World Soul of Azewrath(why else would the Legion version of Azeroth be called Azewrath if not for her being the Harvester of Wrath?!) into her clutches!
Arthas of course won’t seek to revive nor would Sylvanas(who having looked at Arthas through the eyes of Jaina now understands him as someone whose sins got his Soul captured and reduced to a prisoner watching it’s body commit even worse crimes of it’s own volition while the Soul is unable to stop it) but Jaina will be encouraged to revive at Khaz Algar where G’Hanir(which looks like the Ardenweald sitting on top of a Shala’nir-esque place with the real Ardenweald looking like a Winter Wonderland) is connected to with the reveal that the Khaz Algar we visited was the real one even though everywhere else is a flashback.
Vyranoth(who was brainwashed into being Raszageth) has her memories bluntly restored by the Winter Queen who apologizes for allowing the 1st Harvester of Wrath to treat her as being in the wrong for disagreeing with Iridikron and dying in a mutual fight. Vyranoth(who uses the Model she used as “Raszageth” but with Ice Effects) now has reason to want Denathrius dead and joins us in reviving!
We get into contact with Anduin who is now an old man and is surprised by Jaina coming back and needing our help to join society with us being former Arathi Souls caught in that storm.
Ardenweald(a Winter Wonderland) and G’Hanir will be the Zones of Patch 11.1 coming without a Raid while Kezan and Undermine will be 11.2 coming with a Raid.
Azewrath escapes into Reality through the Stonewright’s Dream(after we beat the 11.2 Raid in Undermine)'s connection to Khaz Algar and attacks Azeroth’s Core as War Within’s Final Boss.
The real Xal’atath/Remornia uses the broken Dark Heart(stolen from the Stonewright) on the Sunwell and starts Midnight. Vyranoth spearheads the assault against her through the Protodrakes!
Arthas who has been Anduin to us for far too long and Sylvanas who has been Jaina to us for far too long are left in the Shadowlands never to be seen again.
Such a ridiculous possibility! Serves to smooth over retcons while not getting rid of the events of the Expansions despite changing how they exactly happened! Even so it is almost as likely as the affair the OP is suggesting! Which is to say incredibly unlikely!
Even so I should put this post on Favorites just in case I need to say “Called it!”