This is just my opinion and I know there are a lot of posts about this, but I've been reading theories about Acotar 5 and... I just can't see this book not being about Elain and Azriel.
If we get them, we will have:
A journey to find the Forth Trove;
A connection to Koschei, who is the main villain so far;
A political plot involving the Autumn Court, the Human Realm and possibly Spring Court;
Consequently, we will have to deal with Beron AND Eris;
If Beron gets involved, we have the chance to finally see the true about Lucien's father, which means Helion will also be in the book;
A plot involving the Blood Duel because there's no way it's not coming up;
And of course, if we have Azriel... we get to see our spymaster in action!!!!!! ARRG
For the first time, we get to see a different type of female main character. In Elain's book we'll get to see the "different sort of strength, a better strength";
Correct me if I'm wrong, but it'll also be the first time we get a "forbidden romance" from SJM;
They already have feelings for each other or at least attraction (but I don't believe one bit that a person gets over an 500 years love or an engagement just for attraction) and imagine the angst (and the angst smut... is that a thing? HAHA)
Elain defying the freaking Cauldron;
Elain defying the mating bond;
A story about fate x choices;
If the plot is a cobweb, Elain is right in the center of it. I don't see the next book being about anyone else. And doesn't make sense including a bonus chapter from Az's POV just to the next book be about someone else, so... 🤷🏻♀️
I didn't include how they can help each other to overcome their issues, because honestly there are better posts about that and you could argue that any love interest could help them, not necessarily Azriel and Elain. But I've seen people saying SJM "killed" Elriel in the bonus chapter... but all I can see is an author setting obstacles for an upcoming book.
Feel free to add to the list! This is just my opinion, so pleaseee be kind! 💙