You mean another campaign book in the same setting? Tomas has said in multiple interviews that their plans for expanding FBL is to go in each of the 4 cardinal directions with new maps and adventure sites.
While he hasn't mentioned campaigns for each, I would guess that there will likely be a meta-plot to go with each one (similar to what they've done for the MYZ expansions, Genlab Alpha, Mechatron, and Elysium). I would be surprised if they repeat the core rules from the main book, so I would wager that these expansions will be a combination of the GM Guide and Raven's Purge in a single volume. You'd just need the Player's Handbook to go along with it.
If you are interested in other settings, once the OGL is released I expect you'll start to see many new settings coming out. I'm working on a setting which will likely not have a single big meta plot. Instead, I plan on having a bunch of "campaign hooks" that are each about a paragraph long and spark a GM's imagination, whilst also allowing them to get started straight away. There also won't be a ton of setting info. The idea being that a GM makes the setting their own, though there will still be a setting overview, a bestiary, and a brief history for those who want them.
I personally think this is the way to go for future third party supplements. Focus on the main mechanical changes needed (Kins, Professions, and Talents) and then give an overview of the setting so that GMs can adapt as needed for their own tastes.