The storyline of Sword Art Online : Ordinal Scale movie happened after Sword Art Online II seasonal animation of mother rosario arc.
Some elements from the Ninth novel (which at the time of the film's release had yet to have been adapted into the anime) are explored in the film. for example, the Book describing events during the SAO incident and Asana wanting Kirito to meet her mother are both elements briefly talked about in the novel.
Every time Kirito stumbles upon the "ghost" of Yuna before disappearing, a white butterfly is usually seen for a moment. This seems to represent Yuna in some way.
The story arc about Kirito and Asuna promising to see Shooting Stars is inspired by the opening chapter of the SAO Spin-off novel, Sword Art Online Progressive.
Sword Art Online Movie will be added 3 new characters : Yuna (Ordinal Scale's Idol Diva), Eiji (2nd ranked Ordinal Scale Player, Main Antagonist), and Dr. Shigemura (Augma Developer).