Based on the previous books, I think something that's going to happen mid book 6 is the not necessarily romantic trip to the Middle of Prythain to hunt for an artefact of certain significance.
Rhys and Feyre went for her wedding ring in the weavers cottage in the forest, Nesta and Cassian, with Az, went to the Swamp for the mask.
But if we continue with the theme of great power hiding in mountains; the asteri in the prison, the inmates of the prison, bryaxis in the library, the depths of the court of nightmares, the super magic mirror that Mors family had, then they're going to go back Under The Mountain.
Elaine being able to See things will see that there's a vault hidden in the deep caverns below it, sealed by the high lords of Prythain with the magic artefact that will be important in defeating Koschei and freeing Vassa. Lucien will be all like of course I'm coming with you, Vassa is my friend and I care about this (even though Elaine will have made it clear she's not into him as a mate and they're trying to cope with that). Feyre will be like I respect that Rhys is going to try and convince everyone that they need to help us open the vault, but since I can do it on my own, Hellion I'm tagging you in to help babysit my hellion! While they're under the mountain Elaine is going to be able to See atrocities that were committed there and is going to be like tf, and some things cannot ever be unseen.
Eventually they're going to hit curses Amarantha laid to keep people from getting ideas about accessing the lower vaults (and the useful things there) and one of them will trigger Feyre and she will become basically no use for like a chapter or two. During this time, Lucien will trigger latent spellcleaver abilities and day powers to get rid of the curses and be like wtf wtf wtf. Elaine will promise to keep this secret, they think because Feyre is out of it, she couldn't possibly know the truth. They will either on that trip or the eventual return trip do some magic that ends up cleansing the curses and stains of Amaranthas reign from the space, so that while the shadows may not all be banished, it can begin to be a place to rule all of Prythain from once again, but like in a healthy-ish way
Also the reason why Hellion is the one babysitting is for the wonderful pain it will cause. Due to reasons Lucien will have to return to the river house in Velaris, and he'll have compartmentalized a lot of the feelings about his not being in the line of succession for the Autumn Court and the power impact this has on the chessboard of Prythain politics and be trying to be rational about how to deal with this. But his control over that compartmentalizing will be like spider silk, strong as steel in the right circumstances, but delicate as a wisp when he sees his Daddy Dearest being a doting parental figure to a fae child and he loses himself to what ifs and what could have beens