So there was a Q&A at the LA signing last night, and one of the last questions he asked led to a bunch of interesting facts, including some lovely information about the next book! (Video here, relevant at 57:51.) (I know he starts off saying 16 in the video, but he corrects himself to 15 at the end.)
First off, the two likely titles:
•Bag Man
•Skin Game
Plot information:
•It's a heist!
•He doesn't outright say so, but the implication is that he's heisting with the Denarians. ("The problem won't really be the heist so much as the people he has to do the heist with. Because this is a book number that's a multiple of five, so the Denarians are back.")
•Michael's likely coming off the bench.