My vote is for Fiend Folio, followed by Creature Catalogue. And then more alliterative named monster books! Beast Bible, Dragon Dictionary, Horror Handbook, Ogre Opus, Villain Volume...
I wouldn't want a PHB2, ever. I don't want more classes at the depth that the PHB offers them. Maybe all the Psionics stuff in one book so it can be off to the side for people who want it, but other than that I really would prefer WotC to release books that tinker with the classes as we have them. Each class has a lot of areas where more options can be added without losing the thread entirely, and most concepts should fit as subclasses of existing classes. Some of those should be setting-specific; I would like a book like Arabian Adventures for Al-Qadim that adjusted the existing classes rather than inventing a bunch of new ones. 5e gives great ways to do that, like new magic schools, fighting styles, backgrounds, and all kinds of crunchy bits that can integrate nicely with setting elements.
Maybe at some point I would be okay with a book that took the attitude more of Unearthed Arcana, expanding the PHB rather than trying to add more core classes into it. Except less bad than Unearthed Arcana for 1e was.