There Are Still Too Many Open Plot Threads Around Elain and Lucien
I've seen a lot of theories that the next book will be about Nesta or Azriel, especially because of the ending of CC3. But even though we had the crossover and everything is connected now, ACOTAR is still its own series, and Crescent City is another.
In my opinion, even though CC is written in third person and Throne of Glass has multiple POVs, ACOTAR has always had its own identity with the story being told through the main characters' perspectives. Yes, A Court of Frost and Starlight expanded the POVs a little with Mor and others, and A Court of Silver Flames also changed the format, but I honestly don't think Sarah is suddenly going to turn ACOTAR into a story with a huge cast of POVs. I think she'll keep the style she's been using in this series.
With that in mind, there are still so many plot threads from previous books that need to be resolved. That's why I personally believe the next book will be about Elain and Lucien.
Will Lucien find out Helion is his father? Will Helion find out Lucien is his son? If Lucien doesn't get a POV, whose perspective are we supposed to experience that revelation through? It wouldn't make much sense for such an important moment to happen through someone else's eyes.
The Archeron sisters' father made a bargain with Koschei. What exactly was that bargain? If I remember correctly, Lucien was with him when he went to the lake.
Vassa is getting closer to having to return to Koschei. What's happening with the Band of Exiles? Where is that storyline going?
And even if there's the smallest possibility that Elain rejects the mating bond (which a lot of people believe), I personally don't think that's what's going to happen. But regardless of the outcome, Lucien and Elain need page time together. Whether they accept the bond or reject it, readers need to actually see that relationship develop.
There are just too many open doors for these two.
Lucien has been an important character since the very first book. He lost Jesminda, and we know he still carries guilt and grief because of that, but we've never actually been inside his head. He had a meaningful friendship with Feyre that slowly drifted apart, and I think that deserves some kind of resolution too.
Then there's Eris, who is clearly hiding things. There's Helion being Lucien's father, a secret Sarah has kept for several books now. Lucien is connected to almost every major political storyline in Prythian: the Autumn Court, the Spring Court, the Day Court, the Night Court, the human queens, Jurian, Vassa, and Koschei.
Right now, it feels like Lucien and Elain simply have the most unresolved storylines.
It just doesn't make much sense to me that Sarah would spend three books building up something as significant as Lucien's parentage, only to move on to another main character like Azriel, Nesta (whose story has already been told), or even the Valkyries before addressing it.
I know ACOTAR isn't an extremely political series, but I also don't think Sarah would spend this much time setting up all these storylines if they weren't meant to pay off eventually.