The single thing I'd LOVE to see Pathfinderized is something I only this week discovered was open content and that is the Sanity rules. I always just lifted them from Call of Cthulhu d20 but I discovered they also appeared in Unearthed Arcana with a subtle D&D tweak.
I only ever leafed through the GameMastery Guide but if I remember correctly there were just rules about madness and so on, not the sanity points, sanity checks, mind blasting horror thing from Call of Cthulhu.
This is the OGL version that Monte Cook did for Unearthed Arcana: Sanity :: d20srd.org
--edit--
I just found the d20pfsrd version. It looks like it's part of the affliction rules.
The Incantations in the OGL from Unearted Arcana add an element to d20 that is often lacking. Spellcasting tends to either be story-based, like in Call of Cthulhu (the effects are more story-based than number based) or crunchy (D&D 3.0 and 3.5 have mostly numberical effects for spells. Incantations provide story-based spellcasting and story-based effects. Pathfinder does a much better job of providing spells with a story effect in addition to spells with numberical effects (modify memory is a good example), but incantations can provide a nice element to a campaign.
Let me add my agreement with the Book of Templates and the Advanced Bestiary. Too many templates? Huh? What's that?
I'd also agree with Bastion's Airships...I've seen attempts by other publishers, but theirs was best...if still lacking. Paizo could SO do that right, though!
*HINT, HINT*!
I'd love to see a book on airships and floating cities, if nobody has mentioned it.