The love interests for the Archeron sisters and other characters have already changed a number of times, but not in the classic SJM way, meaning the change wasn't shown in the books.
But we know this from Sarah herself through interviews and statements she has made ever since the first ACOTAR book came out.
Lucien was supposed to be Nesta's mate, not Cassian.
Originally it was planned for Nesta and Lucien to be mates, but SJM realised they wouldn't work out. And before she realised this she had done a bit of foreshadowing for them in the first book by mentioning that Feyre had painted flames for her on their drawer.
She was able to twist the flames' meaning from symbolising Lucien's power to instead symbolising Nesta's new found powers, the silver flames.
Lucien becomes Elain's mate instead.
SJM had said that she decided to match Lucien and Elain as mates so they could heal together.
But it's obvious she changed her mind again in ACOWAR and we now know that Lucien and Elain wouldn't work either. Through Jesminda we know what attributes attract Lucien and Elain doesn't belong in that category.
There are zero interactions between them since they were revealed as mates to indicate that even one of them views the other romantically.
And that's where the third change of LI comes.
Azriel becomes Elain's love interest in ACOWAR through foreshadowing.
In ACOWAR he lends her TT, he's the first and only one to realise she's a Seer and there's nothing mentally wrong with her, Feyre wonders why the Cauldron couldn't make them mates.
In ACOFAS Elain gifts him the headache powder and he laughs so full of joy that Feyre mentions it. He makes Cassian stop eating, just to wait for Elain to come back so they can eat all together.
In ACOSF Nesta notices a charged glance between Elain and Azriel and thinks to herself "His secret to tell, never hers".
I'm sure I may be forgetting some things, but you get my point.
And I won't even mention that infernal bonus pov.
Elain shall wed for love and beauty.
In ACOSF Nesta remembers her mother telling her she would marry for conquest and Elain would marry for love and beauty.
That statement came true for Nesta, and we can assume it will be true for Elain too.
Elain's beauty is described as the kind that brings kings to their knees. And who is the other person that is always described as beautiful? Azriel.
Also, she called his scars "beautiful" and I don't think that's a coincidence.
All that being said, I don't think SJM will let all the foreshadowing she's done for them go to waist just to have a secondary character with no connection to the IC and the main trilogy get a book.
Plus, Lucien too has a new love interest too. Vassa. (Maybe even Jurian, who knows if Sarah decides she wants to write polyđ)