Why not combine The Empty and Chuck? Allow The Empty to power-up a vindictive Chuck. This could allow the return of more characters, as you would have Heaven and The Empty to draw from. Also, it would allow Jack and Chuck to revive characters from either place (I.e. Castiel) . Additional story lines could then be shot in Heaven, on Earth, The Empty, or even Hell. This would also tie in the ending with minimal redirection, just another chapter.