Look I have 3 big reasons to believe Sarah hinting Gwynriel as endgame
There are two colour related instances in ACOSF that come to mind. The first is to show the incompatibility between and Elain and Azriel and the second is to show the likeness between Gwyn and Az.
The first is when Elain enters Hewn City wearing a black dress. There is a large chunk of the page dedicated to how unsuited Elain is to the colour black. It says things like “She looked ridiculous in it” “It wore her” “Leeched the brightness from her face”. Anyone who has ever met Azriel knows that black is his colour/ the only thing he wears, it’s what he’s comfortable and most himself in. And he’s a shadow singer for crying out loud.... And a very dedicated member of the night court. If Elain can’t even wear a black dress without looking obscene how can she be beside and share a life with a shadow singer. She doesn’t appreciate his darkness and they wouldn’t bring out the best in each other.
The second instance is when the Valkyries make the friendship bracelets. Nesta chooses the colours, Blue, white and teal for Gwyn, saying they reflect her spirit. The teal is a reflection of the priestesses eyes. The blue is also obvious as a reflection of Azriel because those are the colours of his Siphons. (This is also a nice link between the two of them because her invoking stone is a similar colour and I can imagine some beautiful imagery/ symbolism or whatever of Azriel not feeling like he deserves his Siphons and Gwyn feeling like she doesn’t deserve her invoking stone.) The white however (this might be a bit of a stretch) I think could show a connection to the white ribbon and after all “Az is the new ribbon.”
This is one that loads of people have picked up on but it’s so important and it’s the right level of subtle/ obvious and totally something that Sarah loves to do in her stories.
So it’s well known that mates are described as having this golden thread/ glowing threads tethering them together. It’s described that way with Rhys and Feyre and Nesta and Cassian. The first time Nesta and Cassian make love (like the emotional one of solstice) There’s a chunky paragraph detailing this golden thread forming between them. And then at the end of Azriel’s chapter we see that this golden glowing feeling has started, just at the thought of Gwyn smiling. Like that’s a fairly big reaction to have to someone smiling. Of course there’s more to it.
I also feel like there’s this thing that Sarah likes to do with Mates where she has one of them save the other in a physical sense and then the other repays it by saving them in a more emotional sense (or the other way round). So Rhys and Feyre - Feyre saves Rhys from under the mountain and in turn Rhys saves her from her trauma. Rowan and Aelin - Rowan helps Aelin to emotionally come to terms with her past and in turn Aelin saves him from Maeve. Nesta and Cassian - Nesta saves Cassian from the king of Hybern and in turn Cassian saves Nesta through emotional healing.
Now with Gwyn and Az we’ve seen Azriel saving her from the temple and I envisage Gwyn in turn to save Azriel in an emotional way and help him accept himself.
Also... Azriel’s shadows are very much in love with her.
They have certainly led very different lives but they have a lot of shared History and trauma which I think could be really reassuring and helpful to the other while they’re healing. I feel like these two would be great at giving advice to each other but never take it on themselves and perhaps being in this situation would make it easier for self reflection.
I think a big one is this idea of being trapped. Azriel was trapped for years forcibly and even though Gwyn was in the library by choice they both experienced darkness with no way of getting out/ moving on. I think that they both appreciate the comfort of the darkness as well as the pleasure of the sun light and I just think this is rather adorable. The idea of seeing them beside one another the whole time they’re healing.
Another branch of this “Shared History” is the fact that they’ve known of each other for years.
When Azriel went to Sangravah he acted differently to how he'd ever acted before - he was rash and killed everyone on sight - I think this is a glimmer of the mating bond. But regardless they’ve probably been in and out of each other’s minds for a few years and when they finally do spend time with each other it feels so familiar and easy. There isn’t past tension, only this new electric feeling which is just so magical.
Anyway I’m in love with them. I hope this makes sense... and yeah... that’s my ramble over.