Lost Song will include a completely original story which will primarily involve the exploration of "Svart Alfheim", a new area of ALfheim Online comprised of a series of floating continents close to the World Tree. The game will skip past the Fairy Dance arc.

For the first time in the series, players will be able to play as characters other than Kirito. However, it has yet to be decided whether players will be able to create their own custom characters or not. Parties of up to three characters may be formed, up from two in the prior games. In keeping with being set in ALfheim Online, players will be able to fly as well as walk on the ground. However, walking within dungeons will be mandatory. Auto-attacks will be disabled while in flight mode, but players may instead target individual parts of an enemy's body as they see fit. In addition, this game will add the ability to use magic to attack enemies or to provide support to one's party. The game will be the first in the series to allow for four-person online multiplayer.
There will be approximately 20 main characters in this game.