Skinchangers is going to be an interesting book. It's actually going to be more of a broad and variant-filled toolkit book than a book introducing "the latest additions to the World of Darkness"; as with other supplements, the idea is to present a book that doesn't require other supplements to be useful, or that other supplements won't come to rely on or reference. I actually would be surprised if any group wound up using all the ideas presented in that book in a single chronicle — there are going to be a lot of ideas, and they're going to go down some interestingly divergent roads.
Anyway, each individual idea should fit nicely with Werewolf, even if it'd be a bit of overkill to throw them all at the players in the same chronicle. I'll talk more about it when the time comes, but right now it's only in the "outlined and first drafts are being worked on" stage.
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Skinchangers will be a book that "relegates" other shapeshifter legends to being "not like werewolves" — they won't be Hosts or Ridden (entirely), but neither will they be races like the Uratha with their own shapeshifter cultures. There are a lot of ways to represent a skinchanger, and the book should demonstrate that you have plenty of options to do that apart from Hosts or Ridden. Hell, you may even find new ways to build wolf-changers that aren't Uratha.
One of the chapters is titled "Skinners and Thieves." And it is a large, versatile chapter with many examples. Consider that, if you will.
It is a Werewolf sourcebook, not WoD — it could have worked very nicely as a WoD book, but more options are available for the book if you presume the readership has some familiarity with the Shadow and spirits.
As for diluting what werewolves are about, I am of the opinion that it's much less likely to do so to any noticeable degree. Remember, as a sourcebook it's basically completely optional; you don't need it to understand what's going on in other books, and the critters therein aren't going to imply any major changes to the cosmology by their existence.
One of the things that I'm enjoying about this book is that two-thirds of the authors — despite being awesome authors who've already done quality stuff for Forsaken — have no prior work experience with Werewolf: The Apocalypse. (The third's an old Changing Breeds hand, but also a proponent of doing something very different this time around.) As a result, these skinchangers are being devised by people who are taking the concept of "skinchanger" as if it was totally fresh, and coming up with ideas as novel as the Mokolé and Rokea were back when they first debuted. In fact, arguably even moreso, because the Mokolé and Rokea had certain things in common that are by no means common to all the critters and misguided people in this book.
This book is, in its own way, something for the fans of skinchanger legends of other cultures. It provides ideas on how to build the legend of their choice. However, I'm also trying to make it a good enough book that reading it will get people to say, "Ah, so that's why Ethan wanted to do skinchangers in this method this time around. Good for him — these beasts are wonderfully, disturbingly cool!"