I know Paizo listens to their fans, so I thought I'd mention some things that I'd throw down some scratch for regardless of gaming system.
Book of Encounters. Paizo's encounters are typically very high quality and sometimes I raid Dungeon magazine just for stat blocks. Their encounters tend to have good descriptions, interesting settings, and fun opponents.
Book of Bad Dudes. The Exemplars of Evil book from WotC was good, and I'd pay to see Paizo's version. They also come up with BBEGs that are bigger, badder, and more evil than most other people's. If they came up with world-shaking opponents (a la Elder Evils but not so crappy), all the better.
Gazetteers of Golarion countries. Something in the same vein as the OD&D gazetteer series for Mystara. IMO, these are some of the best gaming products ever (up there with Paizo's adventure paths) and I'd be interested to see the same level of quality that we get in the Adventure Path modules (overview of Sandpoint, Magnimar, Stone Giants of the Storval Plateau) applied to a whole product.
A 'guide to fluff'. Paizo's fluff is awesome. I think they, and Expeditious Retreat Press, have some of the best methodologies for taking cool ideas and adding them in a very fantastic way to a gaming setting. Maybe something short, published as a PDF, that details different lifestyles of various ethic and racial groups of Golarion, what they eat, how they live. WotC made a stab at this with the Races of * products, but it doesn't compare to XRP's work.
When I look through this list, I think what I want to see as much of as possible are things that make a DM's job easier. Not all of us have that much time and I'd rather spend my few hours of prep time writing interesting stories and coming up with creative ideas instead of statting up a minotaur fighter/cleric 3/7. One of the promises of 4th edition is that it takes less time to stat up monsters and NPCs, and if there were a library of things to draw from I think that would help alleviate the amount of time that DMs spend on this kind of thing.