Warning: Story spoilers ahead for anyone that hasn’t played the game. But it’s honestly very possible angry bros on the internet spoiled it for you already, so read at your own risk.
Many alright heard about this little easter egg, but for those that weren’t aware:
Within the Hall of Justice in Suicide Squad: Kill the Justice League, there is a tribute on the wall in memory of the original Harley Quinn – Arleen Sorkin. Also in the room, to the right of Arleen, is a tribute to Kevin Conroy – our beloved Batman. His final performance as the Bat was Suicide Squad: Kill the Justice League.
It seemed appropriate when I took the screenshot in my game to have Harley in her classic outfit.
The Quinn-tessential Guide to Harley Quinn 