In the A Matter of Family DLC, the Joker will sometimes call Batgirl "Bat-fake", a reference to the insult he called Terry McGinnis in the animated movie Batman Beyond: Return of the Joker.
Before the boss fight against the Arkham Knight in his tunnel excavator, the Arkham Knight says that he asked if the excavator came in black, a nod to the line from the Christopher Nolan film Batman Begins.

The Arkham Knight also remarks how Batman always "gets knocked down only to pick himself up again." This is another nod to a line spoken in the film. Spoiler:Additionally, the Knight says he learned this from an "old friend". Because Alfred is the one who spoke the line in the film, this is also a hint at the Arkham Knight's true identity.
Contributed by Boyobmas
Spoiler:During the scene in which Batman is forced to watch Oracle kill herself, The Joker can be seen nudging the gun she uses closer to her, which should be impossible as The Joker is only a hallucination in Batman's mind. This is actually a subtle foreshadowing to later in the game where it is revealed the entire scene was a hallucination.
Contributed by Boyobmas
A Mac and Linux port were planned to be released and was being worked on by Feral Interactive, with a Spring 2016 release. However, it was later announced on Steam that it had been canceled with refunds offered to anyone who had pre-ordered a copy.
Contributed by KnowledgeBase
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Spoiler:In the DLC "Matter of Family", a hidden room can be found between the Ferris Wheel and the Great White. Inside the room the player can find DC villian Starro the Conqueror in a containment tube.

In front of the Great White is a billboard with another reference to this villain and its "mind control" ability.
Contributed by SOGESNAKE
When the game first released on Steam, Arkham Knight was horribly optimized and panned by customers and media alike. The game was taken down and eventually replaced months later.

However, one running joke was that the original buggy game ran at the speed depicted on a speed limit sign in the game's promotional art, which was 30 miles per hour (or 30 fps on most systems). This artwork was changed when the game was re-released on Steam, now showing a 60 miles per hour speed limit and serving as a subtle nod to the game's performance fixes.
Contributed by RoboBadnik
The scene in whichSpoiler:Batman must escape an exploding Ace Chemicals seems to be a nod to a scene from Tim Burton's 1989 Batman film Spoiler:where Batman rides through an exploding chemical plant.
Contributed by Boyobmas
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During the game's opening sequence Spoiler:in Pauli's Diner, the player can spot Sefton Hill, co-founder of Rocksteady and Director of the game.
Contributed by Boyobmas
Spoiler:During the final hallucination of the game where the player is controlling a gun-wielding Joker, as he guns down Penguin, Riddler and Two-Face, the song "Only You (And You Alone)" by The Platters can be heard playing on a radio throughout the corridor where the villain trio are cowering.

This was the same song that Joker himself sung in the end credits of Batman: Arkham City.
Contributed by MightyKombat
The biggest obstacle when designing the game was integrating the Batmobile. During early development, Rocksteady placed a prototype Batmobile in the existing Arkham City map, and learned that the claustrophobic city designed for Batman to glide and grapple did not work well for driving a vehicle. Gotham City was thus redesigned to accommodate the new mechanics.

The designs for the Batmobile were based on the earlier design from Arkham Asylum, instead of models from the history of Batman comics and media, and evolve that to meet the necessary gameplay requirements.
Contributed by DrakeVagabond
One of The Arkham Knight's game over lines is: "Look at me while you die, Batman. LOOK. AT. ME." The last part is a nod to a line spoken by The Joker in The Dark Knight.
Contributed by Boyobmas
One of the Joker's Game Over sequence is a reference to Metal Gear games where characters reacts on the death of protagonist Solid Snake in the same manner.
Contributed by DrakeVagabond
Returning to Langstrom's Lab after solving the Man-Bat case, reveals the place had been torn (farther) apart, with this wife's body missing, and a cryptic message written in blood on the broken monitor. This may be a clue that Langstrom's wife is still alive and is roaming free as She-Bat from the comics.
Contributed by DrakeVagabond
There are magic shops named Zatanna all over Gotham, as well as a condemned shop called "John Constantine Paranormal Investigater". This is a reference to the magic based heroes from Vertigo/DC Comics, Zatanna and John Constantine.
Contributed by DrakeVagabond
In the chamber where Ivy and Barbara were captured, there are posters for the "Tanaga Dojo" with the instructor "Kairi Tanaga". Kairi is a character from the Batman Beyond animated series who teaches Terry her martial art. She and Bruce trained under the same teacher (who in this universe is Master Kirigi).
Contributed by DrakeVagabond
There are stores named Music Meisters, which is a reference to a villain from the Batman: Brave and the Bold animated series.
Contributed by DrakeVagabond
Under the Logerquist building, there is a store called Cadmus, which is a reference to the DC Comics laboratory that specializes in metahuman research.
Contributed by DrakeVagabond
In the Miagani Island Botanical Gardens, there is a bench with a plaque saying "In Loving Memory... Dr. Alec Holland", referring DC Comics/Vertigo character, the Swamp Thing.
Contributed by DrakeVagabond
In Wayne Tower there is a dog bowl with the name "Ace" labelled on it. This is a reference to Bruce's dog in the comics, Ace the Bat-Hound.
Contributed by DrakeVagabond
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A locker with a name "Ray Palmer" can be seen when visiting ACE Chemicals, with an atom symbol sketched on it. Ray Palmer is the real name of "The Atom", a hero from DC Comics.
Contributed by DrakeVagabond
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During the story mission at Ace Chemicals after turning off the pipes to enter the lower level, just to the right of the obstacle is a clipboard with a vile labelled "Copperhead Poison." This is in reference to Copperhead from Batman: Arkham Origins.
Contributed by DrakeVagabond
If the player has already captured Spoiler:Man-Bat and the internal date of the system is set to October 31st, the player can trigger a jumpscare by him while trying to grapple atop a building. Going back to GCPD reveals that he has escaped.
Contributed by Boyobmas
There are two unused game over sequences for The Riddler that suggest that the player was originaly supposed to rescue hostages from him, similar to Batman: Arkham City.
Contributed by Boyobmas
In Wayne Tower, if Batman checks his answering machine long enough, a message from DC Comics character Lex Luthor will play, followed by a message from a lady named Kate Kane, aka Batwoman.
Contributed by DrakeVagabond
When reviewing the camera footage Spoiler:to find Barabara's kidnapper, Victor Zsasz, from the previous Arkham games, can be seen in footage among the other rioters.
Contributed by DrakeVagabond
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One of the posters seen in Gotham throughout the game is for a film called "The Ghost in Gray" and features a man in a hat and goggles. An actor named "Simon Trent" is also listed on the poster. This is a reference to the Gray Ghost, a character from "Batman: The Animated Series" and the eponymous character of the episode "Beware the Gray Ghost." The Gray Ghost was an old TV superhero played in universe by Simon Trent, with his costume consisting of a fedora hat, goggles and trenchcoat. Bruce Wayne was a huge fan of the Gray Ghost series and character, with Ghost serving as inspiration for Bruce to become Batman. In said episode, Batman enlisted Trent's help (in costume as the Gray Ghost) to stop a criminal who was imitating a scheme used by one of the villains from an episode of the Gray Ghost series.

Additionally in the Batman Animated Series, Gray Ghost/Trent was voiced by Adam West, who played Batman in the 1960s TV series. Adam West's Batman costume is available in Arkham Knight as DLC.
Contributed by MightyKombat
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The screens of the watchtower stations the player must destroy in the side mission: "Occupy Gotham", are actually recycled from the Cryptographic Sequencer's screen from Batman: Arkham Asylum.
Contributed by Boyobmas
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Spoiler:During The Joker's nightmare sequence, the player can spot a painting of Batman carrying The Joker's body from the end of Batman: Arkham City. The painting itself is actually a piece of concept art from Arkham City.
Contributed by Boyobmas
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During the game's true ending, Calendar Man can be seen in the crowd Spoiler:outside Wayne Manor before it is destroyed.

This easter egg ties into a hidden secret found Batman: Arkham City, where if the system's date was changed to December 13th, 2004, visiting Calendar Man would reveal new dialogue.

"The end of days is coming. I was there at your beginning, and I will be there at your end."
Contributed by SOGESNAKE