While I'm reluctant to make too many claims about what House of Sky and Breath might tell us about upcoming books in the ACOTAR series, I think it does make one thing clear: Azriel's book is next.
Let's start with the obvious. Azriel is the one who finds Bryce (HOSAB, 793), and he's the one who wields Truth Teller, the twin weapon to the Starsword (HOSAB, 796).
Anyone who could winnow or fly could have found Bryce at the end of HOSAB. We would have recognized Cassian about as quickly as Azriel, and can you imagine if Feyre had be the one to find her? I don't think that with this moment, SJM was going for the fan screams. I think she was foreshadowing, and I think it's likely that Azriel was uniquely able to detect Bryce's presence, which could have enormous implications.
I know that Nesta will likely be an important character in this arc because of her ability to locate objects Made by the Cauldron, but it's notable that she isn't highlighted in the final chapter of HOSAB. Her powers will come into play, but perhaps they're not central.
Moving on to Truth Teller, we know from House of Earth and Blood that the Starsword needs to be reunited with its twin knife (HOEAB, 458) in order to reunite the Fae. This might mean the meeting at the end of HOSAB, or it could mean more, particularly if Bryce needs to take Truth Teller back with her to Midgard. It could mean that Azriel has to travel with Bryce, or it could mean that Bryce just needs to take the knife with her. If she takes the knife, though, she will likely need to bring it back... especially since it's Azriel's, and this is a character who sleeps with a dagger under his pillow. Either way, this involvement is going to have some impact on his arc. At minimum, Azriel somehow got this very special blade in his possession. It marks him in some way. But it could mean even more.
After all, Bryce has to travel through worlds sometime in the course of the next book, whether to Hel, Midgard, or back to another planet. What we don't know is whether she will go there alone. While any ACOTAR character who can winnow seems like a potential candidate for traveling with Bryce, assuming that that power can link up with the Horn, Azriel seems the likeliest because, as Nesta and Cassian point out in ACOSF, his ability to winnow is different from that of the other characters:
"Illyrians do possess magic on one night a year, though: the night before the Blood Rite, when the war-band leaders can winnow the drugged novices into the wilds. Don't even ask me why that is. No one knows."
"Azriel can winnow all the time, though."
"Az is different. In a lot of ways." His tone didn't invite further questioning. (ACOSF, 191)
Is it possible that his unique method of winnowing, which, based on ACOMAF (240) and ACOSF (349), seems to be tied to being a shadowsinger rather than the usual winnowing power in Prythian, could make him an ideal candidate to travel with Bryce, with her connection to the Avallen fae?
In terms of knowledge and motivation, there are only a few characters in ACOTAR who seem particularly interested in other worlds: Rhys, Amren, Merrill, Gwyn, and Azriel. Cassian points out Rhys' and Azriel's interest early on in ACOSF (37), Amren has a vested interest and firsthand knowledge of other worlds, and Gwyn is helping Merrill research her theories of other worlds (154). I think it's reasonable to assume that these characters are going to be important during the Bryce-in-Prythian arc.
Incidentally, I also think that this is another subtle hint that Azriel and Gwyn are going to be drawn together more in the next ACOTAR books... but let's assume that happens in ACOTAR5, not in what I think will be Bryce's relatively short stint in Prythian.
Finally, in ACOSF, we see Azriel's "shadows curled around him, snakes ready to strike" (101). In HOSAB, SJM describe's Ruhn's shadows in a similar fashion, writing that, "His shadows swirled up his left arm like twining snakes" (57). Not only are their powers similar (though Azriel's abilities are more like Cormac's with the shadows + winnowing and no daemati abilities), but SJM uses the same metaphor to describe them. She wants us to see this.
While Bryce directly compares Ruhn and Rhys, we know that Rhys' arc in ACOTAR is more or less complete. He also has the bargain with Feyre that leaves him vulnerable, making him a bad candidate to go to other worlds or lead an army. Azriel, meanwhile, feels more disconnected from the Inner Circle (see: his bonus chapter in ACOSF; there are no page numbers, alas!) and, as spymaster and Carynthian, is able to gather information while being an active warrior. There are a lot of possibilities for what he could do! Even if it's just passing along information while he hands Bryce Truth Teller.
In terms of other similarities, this is how Bryce describes Azriel's wings: "Great, black wings. A demon's wings." (793) This evokes Hunt's description of Apollion's wings: "Wings rustled nearby -- not the soft feathers of his wings, but something leathery. Dry." (237)
We know that Bryce needs to go to Hel, and we know now that Hel is not a literal hell but rather another world (758). The comparison between Apollion's wings and Azriel's/the Illyrian's isn't an accident. Since we are likely going to see Hel in CC3, this similarity is likely to be important.
Given all the mysteries that swirl around Azriel, his possession of Truth Teller, his interest in other worlds, and his connections to some pivotal characters in HOSAB, it seems really likely that CC3 is going to set up his arc in ACOTAR5. As SJM once said... it seems pretty obvious.
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