User 'Dribbiz' managed to come across this amazing Easter egg in the Batgirl DLC for Batman: Arkham Knight. It features the villain "Starro", from Justice League, who can attach itself to people in order to control them, and can also create clones of itself at will. Starro was the first villain to ever battle the Justice League of America in the comic books, and since it exists in the Batman: Arkham universe, it opens the doors to the possibility that the Justice League probably does as well, or will exist in a future game.