I don’t believe Arthas is a character who would want redemption even if offered to him on a silver platter. The guy basically had an HD plasma screen with a non-stop live stream of himself killing his father, his mentor, his men and his people from the moment after he picked up a cursed blade of his own free will, and then watched himself destroy the land he swore to protect. The moment Frostmourne is broken and he’s dying, all he asks his father is if it’s over, and that he sees only darkness before him.
Not exactly the dying words of a heartless man with no regrets. More like a boy waking up from a nightmare.
With that in mind, if the story team really wants to involve Arthas in the Sylvanas plot that much, his whole involvement in the creation of Sylvanas should be addressed in-game, and not in a heroic fashion. In a hypothetical scenario where he joins the hunt for Sylvanas, he’s not jumping at the call to hunt her down. He’s fetched in the Maw out of desperation as some kind of last ditch effort to help the champion and whatever other forces are allied with us, because he’s a powerful soul or some other excuse used in the story, and he doesn’t even want to go with the champion initially. He flat out refuses and tells the hero to get bent. He’s a broken, depressed mess suffering in his own personal hell, and he’s just taking it because he knows he deserves it.
Eventually he decides to go, not because he thinks it’ll make everything right miraculously, but because… it’s his mess. And he knows it. And after he died, his mess proceeded to cause the deaths of countless people across the world, and then his mess broke the planes of reality and came back to haunt him years after his death. He killed her, he raised her, and what she did as an undead ultimately traces its way back to him and how he treated her. She’s his responsibility.
Sylvanas mocks him, tells him that he’s no better than her. And he says… she’s absolutely right. Killing her won’t make him a better person, won’t erase everything he did as a Death Knight and as the Lich King. Won’t erase the fact that he made her what she was. He says that he’s not going to lecture her on how him standing against her gives him the superior moral high ground - they’re on just about even terrain on that end after everything they both did. They’re just two monsters facing each other down.
But if he just steps aside, does nothing and just stays in the Maw suffering apathetically to what is going on, not only is he shirking his responsibility in his part in creating her, he may be damning more than just his own, already lost soul. He’s dead, he’s paying the price he deserves, but now countless others are subject to suffering they don’t deserve by being shoved into the Maw instead of being filtered to their proper afterlives. It’s not glamorous. There’s no smirking. He’s regretful that it led to this, even though he knows just being sorry won’t fix anything. All he can do… is try his best to help prevent her from doing whatever it is her plan is. Maybe Arthas even hopes for a suicide by cop, and that if she kills him instead it might bring her a measure of peace she was missing since she wasn’t there to kill him at Icecrown. And given Sylvanas’ general disposition, she’s definitely not going to take regret or pity from Arthas well, nor would she be particularly inclined to let him, of all people, stop her.
Maybe when everything’s all said and done, some Bastion-type or Uther gives him a pat on the back and tells him “good job, boy” and everyone who’s playing gasps in horror at the possibility of him being redeemed simply by helping bring down Sylvanas, but he brushes it off coldly, reminds everyone that he helped them do maybe one “good” thing that involved solving a problem he caused in the first place and that there is no triumph in that, and ultimately returns to the Maw willingly, to his “special place in Hell”.
Where he truly believes he belongs.
Of course, this is just my personal hypothetical “what if they really really really wanted to bring Arthas into this” scenario. Ideally, I’d rather they not bring Arthas back at all. There would be no real character arc to speak of, he’d just be a supporting character who shows up in the expansion and then disappears. I would’ve instead made him return in Legion, taking up the spot and arc that went to High Inquisitor Whitemane but dialed up to 11 (with the same broken, depressed mess I mentioned before, on top of being actually mistreated by the Ebon Blade who hate him but he’s there because Bolvar said so and the Ebon Blade and the Lich King have an agreement together). Add the fact that Calia returned in Legion, was involved in the pre-BfA stuff and was actually killed by Sylvanas herself, and Jaina returned in BfA, and all together I think there would actually have been something more interesting than “lol i’m here to kill sylvanas, yay i’m good again, bye” which is… probably what’s going to happen if he shows up again.