This is just a bunch of thoughts regarding @starfata's idea of Hermes experiencing something similar to Apollo's Trials and going from the severely depressed PJO to something more in line with Epic!Hermes. @chaoticdumbassrogue I thought you might be amused by this, so I'm linking you too.
First of all, there's the actual impetus for this. Zeus cast Apollo out and demoted him to mortality using the whole Octavian thing as an excuse; Hermes, presumably, would have to cause at least equal offense for him to be subjected to something similar. Zeus can't go around stripping gods of their immortality willy-nilly, that's a surefire recipe for uniting everyone against him. He needs an excuse.
(There's also the underlying fan theory that Zeus did it in part because he viewed Apollo as a potential threat - the god is very, very powerful, and despite Apollo's self-relegation to a harmless degenerate, the Thunder Bringer is nothing but paranoid. Hermes - Hermes has never once rebelled against his father. He's the 'good son'; he sticks to his responsibilities, he's a hard worker - partly because IMO PJO!Hermes is so damn depressed and he's using work as a distraction.)
If this is the Athenide Twins AU, however, there is the potential for a very, very real excuse. And it could be twofold.
The first is Hermes simply protesting what's happening to Apollo. He and Apollo - have drifted apart over time, but they once were each other's Favorite Brother. And more than that - they were in love with the Athenide Twins. They know the taste of each other's grief. They were - the other was the only person who knew exactly what they were going through, and there is an unspoken solidarity, an unexpected comfort in having someone else who gets it. And now that Apollo has been punished, just as the Athenide Twins have returned - that might all come boiling to the forefront once again.
The second is - it's the Athenide Twins. Zeus canonically regarded Percy Jackson as too dangerous to live on more then once occasion; he does not like the child of Poseidon. If you're essentially making said child a god, a very, very powerful god with allies among the Olympian Council, he is - he might regard it as a potential problem. If Loyalty and Reason are not on his side... well. That's a threat.
And everyone knew that Apollo and Hermes were in love with the fountain-born twins. Real, true love. They would have done - just about anything for them, and now they are back. If Zeus/Jupiter is acting to preemptively stamp out potential threats, if he's worried that his sons' loyalty is to Loyalty and Family instead of himself - that's all he might need to take action against them. (He seems more Jupiter then Zeus in this excuse, I must admit - if we're going by the 'Rome did it's level best to either erase the existence of the Athenides or co-opt them to their own use' speculation).
So Zeus might take that excuse to case Hermes out and down.
The thing is, Apollo's journey was - very much about reclaiming himself. About rediscovering the truth of his identity after so many centuries of avoidance and lying about things to himself. About coming to terms with who he is and who he wants to be - suiting, for the god of truth. Hermes' would be substantially different.
Hermes is the god of lies. Of journeys. And I think he's been ignoring those domains for a long, long time, subsumed beneath his role as god of messengers and messages. That's - mainly how he's portrayed in PJO; the eternal mailman/deliveryperson. Historically, that is not who Hermes is at all. I think his journey is a journey. (He's forgotten how much he liked traveling. How much he loved the journey over the destination. How it felt, to dance on the edge, with only his wits and his lies charting him a safe path to his goal. And oh, but oh, it is glorious.)
And remember how PJO!Hermes told Percy that he doesn't believe gods can change?
Apollo's quest was about discovering the truth of himself. I think Hermes' might be the realization that he can change. (And that would break him, more than just a little). Where Apollo is rediscovering responsibility and accountability, Hermes is learning how to put down his burdens, how to stop using them as an excuse to punish himself. He's rediscovering his whimsy. (And yes, I'm using this as an excuse to reinvent him as something very close to Epic!Hermes - I like that guy.)
In one way, I think the lesson they learn is very similar - 'you screwed up. You can't change that, but you can do better. So do better'.
I don't know what the end goal of Hermes' quest would be. He doesn't have any legendary antagonists like Apollo does, and besides, head-on confrontations is not what Hermes is known for. He's the god of loopholes, of coming at something sideways. Maybe it's something about dismantling the Triumvate's Empire even as Apollo goes for the decapitating strike? Mm, doesn't feel like it - I do think he'd have to pull off at least one heist with only mortal capabilities. I do think that his and Apollo's paths wouldn't cross at all - their journeys are about them, they wouldn't coincide.
Most of all, I think the journey would have to be about Hermes himself. It can't be about Arsinoe, or 'winning her back' - it has to be something he does for himself.
