Easter Egg: So many in the Rocksteady-developed games, with some of them being extremely well hidden. The most notorious?
Arkham Asylum featured a secret room containing plans for the Arkham City complex. It was so well hidden that players never found it until Rocksteady revealed its existence in a podcast six months after the game's release.
Arkham City featured a similar room, which Rocksteady anticipated would remain hidden for some time but was instead found within the first week of the game's release. However, the studio was adamant that fans hadn't found all of the game's Easter Eggs, and maintained this stance until 2015, when it was discovered that talking to Calendar Man with the console set to the day Rocksteady was formed would prompt a Leaning on the Fourth Wall instance, with Calendar Man talking about the studio whilst never specifically referring to them. For those counting, that wasn't discovered for over three years after City was released.
From the DCAU: As noted above, the first two games see Kevin Conroy and Mark Hamill reprise their respective Batman: The Animated Series roles of Batman and the Joker. Asylum also sees Arleen Sorkin reprise the role of Harley Quinn. While none of the three return for Arkham Origins, it does see two more reprises from the the DCAU: Robert Costanzo as Harvey Bullock and in The Stinger, C.C.H. Pounder as Amanda Waller.Blackgate also sees Adam Baldwin reprise his role from Justice League as Rick Flag, in addition to Pounder as Waller. Conroy does return as Batman in Arkham Knight and Assault on Arkham, the latter also seeing Jennifer Hale as Killer Frost and Pounder as Waller yet again, and the former has Hamill back as the Joker. Underworld not only sees the return of Conroy and Pounder, but also sees Professor Achilles Milo reprised by his second DCAU voice actor, Armin Shimerman.
From Batman: Year One: Grey DeLisle reprises the role of Vicki Vale from Year One in City and Origins (the former being released the same day as Year One). Origins also sees the return of Jon Polito as Gillian Loeb.
From Batman: The Brave and the Bold: Blackgate sees Gary Anthony Sturgis return as Bronze Tiger. In addition to Frost, Jennifer Hale also reprises her role from the episode "Chill of the Night!" as Poison Ivy in Assault on Arkham.