* An option book: many good rules or ideas didn't made to the beta. A book that features the best of these rules, fully compatible to the final PRPG would be cool.
Is it cold under that stone? The one you're under if you haven't seen the thousand posts about a Pathfinder optional rules book.
Nah, just kidding. But this does come up all the time. I think this book is going to be done.
* The Tome of Horror/Pathfinder mega almanach. Featuring all monster from ToH I,II,III completely pathfinderized
Problem is that you'd need a weapons license for that book. And as far as I know, books of that size will probably cost an insane amount of money.
And, of course, it would be unfair of Paizo to do those books themselves. Let the original crew do it.
* Although not covered by d20 system ref, I really like some of the newer core classes like the warlock, spirit shaman, and some others. Maybe there is an possibility to create such classes, and maybe many other core/prestige classes in a class book.
Some of those classes would be quite problematic, since none of it is open content. Most of the names cannot really be protected, and the concepts could be up for grabs, but they cannot copy them too closely.
I do think that there will be a book, eventually, with more core classes. Blackguard will be one of them, I think.
Although Golarion is a really nice campaign setting, I would vote for keeping the rules free of campaign specific material. Instead keep it general.
That's already the plan. Look at the beta. The only thing about Golarion in there's the list of deities, since you need some pantheon in the core rules, and the D&D one is obviously out.