*sighs a bit* Okay. Guys. I have been asked this question a lot, and answered it a lot. I don't know how to give a better answer — Dru & Ty&Kit share significance as main characters — so I guess I'll talk a little about comparison and structures.
First, all series have different structures. I don't think it's super useful or predictive to try to map an upcoming, unknown book series onto an existing series. In TLH the main character was Cordelia, everyone else was secondary to her, and people's roles and the significance of them altered from book to book. It was a big ensemble cast and they mostly stayed put in London especially in book 1.
TWP focuses on a smaller group of people. It also has a very different structure. In book one, Dru is not with Kit and Ty. They are in different places, both of which have their own stories that are significant to the plot. There is no way to see Place One without following Dru. There is no way to see Place Two without following Kit and Ty.
I know that TWP is a long way off. I know there are people who are very angry with me that there's such a gap, but there isn't anything currently I can do about that, or about the fact that I don't yet have the schedule for my upcoming books. That rests in the hands of several different publishers who must coordinate the release times and production schedules for four different series. I am not withholding any information about when these books come out. I simply don't know it yet.
I understand that TWP being a long way off makes for anxiety, and that those who are worried Kit and Ty will somehow be secondary are looking for tiny clues in microscopic details — micro-reading the of placement of the word "and" in my newsletter and such — that are meaningless, but I get that it all comes from anxiety. (FTR, those worried Dru will be secondary are equally anxious.)
I think there is only so much I can say. Because there's a big gap between TLH and TWP everything I do say or every image or hint about it is freighted with a weight of assumption it can't really support. Anxiety is always going to trump reassurance. And truly, at the end of the day, if you only care about Kit and Ty and find the idea of a Dru story tiresome, you will feel like they got shafted because when you absolutely hate a plotline, you will always feel like it's taking up way too much space. That's just how our minds work.
I've been doing this long enough that I know no book can survive a hostile reading. I know that Book Three of a trilogy is the one people hate until they don't. (When Clockwork Princess came out people hated it so much I considered quitting writing!) I know that it's wonderful to love a character but can also be a problem for people when I put out books that aren't about that particular character or dynamic. I know that for a lot of people, Sword Catcher and Ragpicker King are just tiresome things that have no business on my schedule because they're not Shadowhunter books. And I get it. But I also have to block it out, because I've been writing a long time, and I've gotten to a point where I know that I have to write the thing I want to be writing, because if I don't, if I sit down and try to force myself to write something I'm not feeling like writing at that time, I'll be making myself physically and mentally sick. And that's no good for anyone, really.
I suppose the positive thing is that, while this would not have been true five years ago, I am at the place where I want very much to be writing Wicked Powers. I missed these characters and am glad to be back with them. I consider this a story in which there are three main characters. And that is all I can say right now because it's all that I know.
(And this was much more of a general response to a lot of things than a specific response to this question, but I did feel like it was stuff that I needed to say. Creators are at the end of the day, just people. Sometimes we are powerless to reassure. Sometimes we are tired. Sometimes we are wrong. Sometimes we try things and they don't work. Sometimes we can't explain to you what our story is going to make you feel, because only reading it is going to tell you that. This may be one of those times.)
