I have a lot of work done on three things: New Classes and a Role, Items, and Monsters. The three things combined will almost surely be too much for one book, so my plan is to publish two: one that is essentially player-facing, and another that is essentially GM-facing.
Now, along with the Monsters, I was thinking of presenting a setting that is about hunting monsters and magical creatures and using their feathers, scales, hearts, and etc. to make magical weapons, armor, potions, and other items. Thus, combining them into a single "monsters and items plus a campaign setting" book seems completely sensible.
However, this leaves the book that was intended to be player-facing with no content other than the new Classes, and while they certainly form the heart and soul of a book all by themselves, they aren't quite enough to fill it. A big chapter on items and item-creation rules would certainly fit well here, too, but splitting the items and loot chapters between the two books seems senseless.
The obvious answer is to craft a fourth pillar and make it essentially player-facing so that I can do the monsters and items together along with the campaign setting that uses them both, and then do the classes plus whatever other material for the other book. The problem with that obvious answer is that I don't really have a good answer for what other content players need... maybe rather than one big thing it could be a series of smaller things: a collection of new Kits, several sets of Origins divided by setting, new feats, a bunch of pre-generated characters divided by setting, rules for coming together as a party to make a home base or a company, etc.
This is still a ways out - I've got plenty of writing to do still on the items, setting, and monsters, and the more playtesting the new classes get, the better. (If you haven't checked them out yet, look at the post below this one!) If any of you have any cool ideas for content you'd like to see (especially player-facing content that would go well with the new Classes), I'm open to suggestions.