Okay heres my 2 cents of what I expect of TWP (The Wicked Powers) be:
01: Janus will join the Cohort. He is the perfect person to be the head example of the movement. He is literally Jace (and Jace being a all time hero as is), but one that lived in a world full of demons controlled by Sebastian. The one who "suffered" in Sebastian hands. The same Sebastian that everybody believes that his evil came because of the demon blood Valentine put in him. He will make a story of his suffering, about how tragic his life is, about how everybody died because of demon blood. And that will make him quickly rise thru the Cohort. BUT he doesn't believe in the movement as is, his only goal is the benefits he can have, the power he can have (for then, completing his other plans)
02: Ash will be on Janus side for a while. After all, Janus was the only person in Ash's life who didn't treat him as shitty.
03: Janus will kidnap Clary. Being locked in Idris would be a perfect location to put Clary. Nobody can enter there. Nobody has information about what is happening inside. Nobody but Livvy. As we seen in "Ghosts of the Shadow Market" ("The Lost World" story) we can see that she has some powers. And she could hear inside Idris wards. Her power may grow.
04: Ty will be in a investigation and needs Kit help. As we know, Kit is the only other person who can see Livvy. And if Livvy is the key for a investigation, it can be only something Livvy can do. And Ty needing Kit help in a investigation is already confirmed by the author.
05: Double church! That's it. If we have 2 Jaces, why not 2 Churchs?
06: All Princes of Hell will be united. As we seen also in "The Lost Book of the White", Sammael call all the Princes of Hell to a meeting, for then to united to a common goal. And this goal may be putting a end in the war between Angels and Demons all together. To finale try to work together for their win.
07: The book tittles is NOT what you expect! The already announced tittles are "The Last King of Faerie","The Last Prince of Hell" and "The Last Shadowhunter". The saga name being "The Wicked Powers". When we see all this information, the first thing that come to mind is who is the most powerful Faerie, Prince of Hell and Shadowhunter; and what will happen to then. But in reality I believe that those tittles are about power-play in political ambiance. The first being about Ash-Kit (Seelie Queen-King Kieran) and all political mess that is Faerie courts is in the moment. The second is about all Prince of Hell united and all the drama is in the middle of that (as for example, if some warlocks will join then? In what side vampires and werewolves will be? Lucifer?). And the third, all the situation about Cohort and the Clave being out of Idris and all that mess being solved, at same time that Seelie/Unseelie is as mess, all Princes of Hell are together and as usual, Shadowhunters without any other help from the Angels.
08: Emma dies. Based on this snippet, I believe that is about Emma dying. It could have been in the original "The Dark Artifices" saga, after all, is a post from 10 years ago. But there is another snippet from 10 years ago that is accepted being a Kitty one. So who knows (link here, kitty one)
09: Tessa dies. I got the spidey sense from this part at "Clockwork Princess". It would be a good resource to be Ithuriel to fight some Prince of Hell.
Also this part of "Wedding" an extra from The Mortal Instruments: