Sword Art Online's 18th light novel shipped in Japan on August 10th. It's the fourth volume in the War of the Underworld sub-arc of the Alicization arc, as well as the final volume of the Alicization arc. While that was something of a potential stopping point for the series, the novel ended with the notice "Kirito will return. New, completely original Sword Art Online chapter to begin in 2017." The first post-Alicization novel is now scheduled for February.
SAO 19, Moon Cradle is slated to be released February 10, 2017. This one evidentlly isn't the "completely original chapter" since it appears to be based on the Cradle of the Moon web side-story.. written to be readable as a stand-alone fantasy that happens to have the same characters as SAO - where Kirito becomes the Representative Swordsman of the Human Empire Unity Council, now at peace with the Dark Territory (a kingdom of goblins, orcs, etc).
SAO volume 19: Moon Cradle preview (Feb 2017) pic.twitter.com/kAS7Fyl2HS— Takazuki (@Takasadasan) December 10, 2016