During the latest T&G show episode James Swallow revealed that he will write the next Titan novel, which will be part of the The Fall series. ETA
: Just to clarify: James Swallow was in the chatroom during the show, not a guest on the podcast.
, the relevant parts starts about the 67 minutes mark IIRC; BTW I think Nick confused Dayton's upcoming TOS novel with his entry in the The Fall series during that discussion)
So it seems there's only one slot left open with DRG III , Dayton Ward, David Mack and now James Swallow attached to it and it being a five book series. Maybe Kirsten Beyer's next Voyager book?