So heres my thought. We've had a players book (Inquisitors Handbook) a couple of GM's book (monsters and opponents in CA & DotFG) and an adventure (PtU). We know we'll get more adventure books for Haarlocks legacy. But whats missing is a book expanding on the world of the Calixis sector. Heck, it could even tie in to Rogue Trader- do a book of Calixis worlds and say "but of course you can drop them into any other sector for your Trader campaign to visit- just change the name!") .
So thats my theory- 2009 we'll see a book expanding and detailing some more of the worlds of the Calixis sector. Heck, you could even have Subsector books to really plumb the depths of the Josian Reach or Drusus Marches.
Steve