I doubt Blizzard is going to let a character like Varian just go off into the Sunset. I have a theory based on absolutely nothing except what I can pull out of my own arse: Varian dies at the Broken Shore invasion but gets resurrected by a Val'kir.
He opens his eyes, waves an emaciated and desiccated hand in front of his face in panic and slowly looks up to see Sylvanas sneering down at him in malicious glee.
Firstly, one thing is guaranteed; It's not in his character to switch allegiances. He'll make the best of his circumstances or something and join up with the Argent Dawn for a while.
Later, one of two things will either happen. First option is that long-term, maybe next expansion (or this one), Sylvanas will finally make that move that's too evil for everyone else to stand. She'll end up getting deposed as head of the Forsaken and the role will be taken up by Varian Wrynn who will refocus the racial identity towards the idea of finding a cure to undeath and basically not being a bunch of evil dicks. This will remove any worry story-wise about the Forsaken becoming a second Scourge.
The second option is that after his resurrection something happens in Legion's story where the undead Varian makes the ultimate sacrifice to save either the player character or a major lore character from the Argent Crusade. The end result is he ends up being called by Odin to the Halls of Valor. At this point he could either return to Azeroth and lead the Alliance, or remain with Odin as a redeemed and non-undead-looking-type-warrior-person free for use in a later expansion... But still letting Anduin hold the reigns as King of Stormwind to progress his story.
Thoughts? Am I way off-base or is some of this in the realm of possibility?