I'm a fan of splat books, provided the give me something I want. That being said, what stuff would you want from an AoO Supplament book. Here are some of my thoughts.
Organization Primers. Books that flesh out the organizations presented in the book. How they are structured, any coloroful personalities, etc. This ranges from a Pandora Primer, to the "Oblivion Handbook for new Recruits" to "Organizations of the pacific rim".
"Q"-Branch. Gear and resources book. More One-shot devices, vehicles, other spytech.
Book of Legends. This could be a good place for starting a jumping off point on much of the horror stuff. A small collection of real-world legends, myths, and superstition from around the world, and how to fit them into the AoO world. What really is a Banshee? Djinn? Effreeti? Did you know Kroeans think that sleeping in a closed room with the fan on can kill you?
Edit: Removed offensive words.