I bet that approach will be used for additional titles in this series, and figure that's why they've left a lot of the backstory of the game world so vague / open-ended. I hope (and am betting, with reviews this glowing) we'll be seeing additional titles and DLC in the future.
I am waging everything I own we won't be seeing "several titles" in the future.
Dark Messiah AND Arx Fatalis both had a great open universe with alot unexplored, they never created new titles within those universes.
Smith and Colantonio both seem from what they've said, that they like to create a game with a huge echoing universe and leave it at that
