Thanks for the replies!
And I agree that customers hate 'filler' that adds to page count - I was probably more concerned if it happened unintentionally. A horrible example being something like changing the Ninja's 'Ki' to 'Hatred' but leaving everything else the same. And would there be any issue with having a new class with a spell-list of names that reference the official spells?
I don't plan on doing a standalone gamebook. It's layout would be something along the lines of:
- Introduction to World and Setting (my own work that I don't want to be OGL)
- Creating a Character (classes etc) (OGL)
- New Skills/Feats/Spells (OGL)
- Rule modifications and Game Master guide (OGL)
- NPCs and Bestiary (OGL)
- Sample Adventure (OGL)
It'd be great if there was some sort of Pathfinder World Builder template out there that would give a framework to work within - that included the legal documentation necessary should you be doing things like creating new classes - but that's in a perfect world. I suppose the next best thing would be to see how others have done it - any recommendations that closely match my layout plan would be very welcome!