There two ways Blizz could do it. Either a deep and engaging quest line that is Alliance specific. Or in the pages of a book so next expansion Turalyon is gone and he’s just sitting on the throne like nothing happened.
But, I would prefer that do to time travel shenanigans we get the older and battle hardened Anduin from the “Son of the Wolf” comic because tired of the angst Blizz is dropping on him.
Not at all
What if he just… didn’t?
Married to and dominated by Sylvanas 
I think it’ll be the former, personally. Alliance-specific quest to see their king once more, and Turalyon gets MAD. He had a form of power, and now it’s being taken away from him, and he goes fanatical. Leads into light evil expac, where I shall main Shadow Priest!
Same way Arthas did when he returned to Lordaeron and met up with the king at the end of the human campaign.
And he could even put the blame on what he did because he had been possessed by Arthas. Which would cause the Alliance to fraction as they fight him to stop his madness! But would never happen because that sounds like the Alliance would get some juicy drama.
Anduin will appear at the end of Dragonflight. He’s going to assist Wrathion in becoming the new Black Dragon Aspect and their friendship finally blossoms into something more. Wrathion may have rewarded us with epic cloaks of friendship, but he’s been saving a new legendary cloak for somebody special.
honestly we need a new set of heroes to be in charge. we keep having these old ones that aren’t growing or doing anything meaningful.
anduin could be change but hes so messed up at this point that idk if he can even function in any true way for the best of a faction. and its a going to take probably a few expansions worth of time before hes able to do much
Anduin could wander back to the throne and nothing would’ve changed.
Still not sure what’s the point of Turalyon’s leadership and seven years of peace timeskip during Shadowlands. 
The writers/devs probably could have put Genn on the throne but since he’d probably be to busy seeing Gilneas rebuilt, they decided to go with Turalyon.
Amnesia.
He shouldn’t, we should weld a giant manhole cover over the entrance to the maw and for good measure destroy every portal to oribos while we are there.
He’ll come back with Aerin Stonehand as his polterwife.
NO! that is all
He should strut into the throne room, mind control Turalyon off the throne with a bit of shadow magic, and use Turalyon as a MCed footstool while legally changing his name to Manduin…
In an urn. All hail Turalyon! Zealot leader of the alliance! First task? The eradication of the horde! Then the concreting of silvermoon to make way for a new theme park!
BFG 10000 style
That was the true end game of the Chains of Domination campaign.
Ewww. I think it really stinks when they put it in a book. You shouldn’t nickle and dime loyal fans like that.