I don't know why this isn't talked about much, but, among the loose ends and problems that are going to be tied up in TWP and BBOTD, the one we've least talked about is Shinyun Jung.
She is a valid problem too, y'know? And as far as The Lost Book of The White tells us, her current power is comparable to that of LUCIFER. And we know that because she said at the end of the book that her power won't be dependent on any world like Sammael depends on Diyu. That makes her more powerful and more dangerous than the frickin' Father of Demons. She is FAR from gone.
Although I think that her power would be unstable due to the lack of a dependable resource and it would also be dangerous to herself, that doesn't mean she won't do enough damage on the way down.
In one of my reblogs, there's a theory that states that Alec survived a jab from the Svefnthorn because Lucifer bent the thorn's magic. That theory is solidified by the Epilogue of LBoTW, where Sammael gathered the other Princes of Hell, to inform them about Lucifer's plan. Obviously, that means Lucifer wants to keep tabs on the leader of all Shadowhunters, right? And who better to give him personal information about him than Shinyun Jung? Who spent quite a lot of time with Alec and Magnus in RSoM.
She won't be planning on serving Lucifer, though. She'll instead concoct a partnership with him, where they both are on equal levels. In my imagination, Lucifer probably bosses the other Princes into serving Shinyun too. Irrelevant, but still. :)
I also think she's going to be a part of Lucifer taking possession over Alec, which I think was the purpose of saving Alec in the first place.