one of my favourite aus to play with is Ethan getting chucked down straight to Tartarus by Kronos in tlo, staying there for a year, and then helping Percabeth when they end up in Tartarus during hoo. And then they naturally drag him back out with them.
anyway, I think this would be very fun in conjunction with swm. Al is grieving Ethan and takes his frustrations out on Percy and Annabeth because he blames them for Ethan's death, but they're also the only people he really has anymore and therefore the only people he can take comfort in, but hates himself for it because he sees it as a betrayal. Meanwhile, Percy feels really guilty for all the people he couldn't save or got killed during the war, especially Ethan, and is taking care of Al in part to make up for that, and Annabeth has lost so many old friends from camp to this war and she doesn't want Al to contribute to that number.
So anyway, Percy goes missing, Annabeth and Al freak out, then Percabeth do their Argo II shit (maybe Al joins them, maybe not), and Percabeth fall into Tartarus. It's pretty shit at the beginning but then Ethan, who has just been minding his own business down here for the past year, sees a kerfuffle in the distance and takes a moment to get over his bafflement that no matter where he goes or how dead he ends up, those weirdos will find him and drag him into their bullshit.
So he steps in to help them out, partially because he owes them, partially because he hasn't seen another not-monster in a year, and partially because he wants to find out what's been going on up on the surface since he fell. They're both quite stunned to see him but soon fill him in on the Titans' loss, the new cabins, Kronos dying, and also the fact that they're dating his ex-situationship who misses him very very much. And also Gaea is trying to destroy everything now so if he knows where the doors of death are, that would be great.
Ethan agrees to help them is fairly happy with all the developments (except for the Gaea thing). Percy and Annabeth don't go into detail about their relationship with Al so Ethan just thinks "damn, it's great that he was able to move on and find love with someone else, even if it really hurts and ngl kind of kills me inside that I'm not the one he wants anymore. oh well :/", fully unaware that Al would launch himself at Ethan in a heartbeat if he was around. Also maybe Ethan can bond a bit more with Percy and Annabeth while he takes them for a tour. It would be nice for him and Annabeth to put aside their differences and bond over how much Luke meant to them and how much they miss him, especially because it's quite difficult for Annabeth to grieve him openly when he's a bit of a non-person in CHB post-war.
Anyway, they do the doors of death thing and Ethan tries to stay behind and fight because he succeeded in getting a cabin for his mother and helping the heroes of the current conflict by getting them to the doors so his work here is done and he's also still kind of butt hurt about everyone 'moving on' from him but Percabeth are not letting that fucking happen so they just yank him in with them.
If Al joins the Argo II in this au (maybe he wanted to stay out of it because he was being huffy and then heard his bf and gf got sucked into Tartarus so he begrudgingly joined the crew to get them) they can have their reunion when the lift comes up. Ethan would be pretty tentative about it because he did use his last moments to betray the army he and Al were both part of and also he knows that Al has his two shiny new partners and presumably doesn't care for Ethan after being strung along him for the year they were in the army together. His fears are not assuaged by the amount of time Al just stands there with his eyes popping out because he was NOT expecting Ethan to pop out with his two new partners and is currently in a medically worrying state of shock. As soon as he gets over that though he can put Ethan's concerns in the grave and the four of them can skip off into the sunset together.
(reunion can also be done post-war after the gang gets back from questing)