A listing for Batman: Arkham Knight appeared on the GAME retailer site. While no longer available to view, the game was listed for PS4, Xbox One and PC. A Google search limited to GAME's site shows the listing existed in some capacity before being pulled by the retailer.
Update: Game Informer revealed that Batman: Arkham Knight will be the cover of its April magazine. The game will be the final one in the Arkham series from Rocksteady, and is set after the events of Batman: Arkham City. The game will feature the Batmobile and a new area of Gotham City reportedly "far larger" than that of Arkham City. Batman will spend his time in the game hunting down familiar foes: Scarecrow, Two-Face, Penguin and Harley Quinn in particular.
Update 2: WBIE issued a trailer for the game and a press release, the latter found after the break. The publisher noted that the game will launch this year.
Update 3: The game's website lists Harley Quinn as a playable character via a pre-order bonus. The add-on content will include four challenge maps and give players the opportunity to use Harley's "unique weapons, gadgets, and abilities first-hand."
Update 4: An image on GameStop promoting Batman: Arkham Knight's Harley Quinn pre-order bonus lists an October 14 release date for the game. Given that GameStop is the only retailer with that listed date at the moment, we wouldn't mark our calendars just yet. We've added the image after the break and reached out to Warner Bros. Interactive Entertainment to confirm the date.
Update 5: A Warner Bros. representative told Joystiq, "Batman: Arkham Knight is scheduled for release worldwide in 2014. "