Well, "the engine" is really a collection of modules working together, and several of these modules are highly streamlined for a single-character game. Even if we use this engine for Book II, several modules have to be rewritten to support more party members and a traditional turn-based system.
Everyone should wait until after playing Book I before we discuss what to do for Book II. We have some cool ideas for engines, but now is not the time to discuss them.
[edit] But by all means, please continue to share your thoughts here for the future of the game. We are taking notes.