Starting at about 56:30 into the video, Diana says,
The betting at the moment is that [the next book] will be a collection of three novellas, to be published in 2016.One of those novellas would presumably be "Virgins", the Jamie-and-Ian story, which is the only one of Diana's shorter pieces that is not yet available as a standalone e-book. The second novella is likely the story about Hal and Minnie that Diana recently started working on. And I have no idea about the third one, yet.
The idea is that Book 9 would come out with Season 3, if we get a Season 3, and if we do, that would be airing in 2017.I was really startled to hear that Diana is aiming to finish Book 9 by 2017! If it happens, that would be the shortest time between publication of the main OUTLANDER books since DRUMS OF AUTUMN was released in 1997.
When I asked Diana (somewhat tongue-in-cheek) on Compuserve whether she was planning to clone herself in order to get the book done that fast, she gave me a very detailed reply. Among other things, she said she's planning to cut way back on traveling and public appearances in 2016, presumably so that she can concentrate on writing.
I would encourage you to read Diana's explanation in her own words.
Finally, a word about publication dates: Please keep in mind that they are never set in stone! We saw that with WRITTEN IN MY OWN HEART'S BLOOD, which had a last-minute delay in publication, at Diana's request, so that she could get the book finished to her satisfaction. I'm sure that she will do her utmost to get Book 9 completed in 2017, but there are no guarantees.