ACOSH Theory About the Inner Circle
So the new blurbs have definitely ramped up speculation about there being a betrayal within the Inner Circle.
My guess is that there's gonna be suspicion of an informer or something like that working within the IC, or someone going rogue against Rhysand's authority.
I don't think Nesta will be the traitor, but I think fingers will point towards her, with the actual traitor perhaps exploiting the fact Nesta's relationship with the IC is strained in order to pin stuff on her and cause division. I think this will climax in Nesta leaving/fleeing from the Night Court, and going to the Prison with Gwyn and Emerie to rediscover the Dusk Court in A Court of Forgotten Melody.
I think the book will end on a cliffhanger with the real traitor being somebody everyone trusts, and perhaps overlooks, like Elain or Mor, possibly being manipulated/enchanted by Koschei.