As the Arrowverse keeps expanding the Batman mythology throughout Batwoman and the other shows, more villains have been teased despite not making a full-fledged appearance just yet. When The CW announced in 2018 that Kate Kane would be coming to the ever-growing franchise for that year’s Elseworlds crossover, it opened up the door to a major part of the DC Universe. Previously, the DC TV shows were restricted from using or even referencing characters from Batman's corner of the universe due to Warner Bros. Studios’ hard restrictions on referring to anything from Gotham City. It probably also had a lot to do with Gotham, which at the time was airing on FOX.
But once Bruce Wayne’s famous cousin came to the small screen to join the Arrowverse, the Gotham City realm was wide open, and with that came a smorgasbord of references to various Batman characters. While Batwoman has primarily been focusing on Alice (Rachel Skarsten) as the primary antagonist, other villains from the Dark Knight’s rogue’s gallery have been established and had their statuses declared in one way or another.
Currently, there is some uncertainty within the Batman corner of the franchise, given that Ruby Rose is exiting the series as Kate Kane and Javicia Leslie is taking up the mantle as a new character. Whether or not this will perhaps see Batwoman introduce some of those iconic villains on-screen remains to be seen. Until Batwoman season 2 arrives, this is every Batman villain that the Arrowverse has referenced but still hasn’t used yet.
Harley Quinn
One of the biggest villains, whose "cameo" became a famous moment in the Arrowverse, was teased in Arrow season 2 when Harley Quinn made her verbal debut. While fans never see her face, Tara Strong's voice is heard during the assembling of the Suicide Squad. Best known as the Joker's famous accomplice, Harley has over the years evolved into her own independent character that is also a member of Task Force X. As far as her relationship with the Joker in the Arrowverse goes, little has been revealed so far about their dynamic. Aside from appearing in a deleted scene later that season, Harley has not even been referenced in years, not even in Batwoman.
Mr. Freeze
While his wife Nora Fries appeared in the Elseworlds crossover and wielded his infamous ice gun, Mr. Freeze was actually established a couple of episodes before the 3-part event. In The Flash season 5, Thomas Snow reveals that he worked with Victor due to his expertise in cryogenics. Even though he doesn't take part in the 2018 crossover, Freeze seems to have at least been in Arkham Asylum at some point given that his ice weapon was there for Nora to steal. While he is definitely operating as his villainous ego, it's unclear if he is still in Gotham City or if he is locked up somewhere.
Catwoman
During Elseworlds, viewers were treated to multiple Easter eggs when the heroes went to Arkham Asylum to get hold of Dr. John Deegan. At one point Selina Kyle's cat-ears are seeing lying on a shelf, with a bag belonging to "S. Kyle" confirming that Catwoman was, at that point, locked up. Since Kate took over the title as Gotham City's protector, Selina hasn't been referenced once. It wouldn't be surprising, though, if Catwoman used her many skills managed to escape Arkham at some point. If Batwoman ever were to get access to this character in later seasons, Catwoman would make an interesting player in the series given the nature of her being an anti-hero in a lot of Batman stories.
The Riddler
Edward Nygma a.k.a. The Riddler is another famous Batman foe who has, through the Elseworlds crossover and later Batwoman episodes, made it into the Arrowverse. During the Arkham Asylum scenes, Nygma's name can be seen on one of the cell doors. Additionally, his Riddler-cane is also spotted amongst several other items belonging to other iconic Batman foes. The biggest call out to Riddler happens a year later when Tommy Elliott (who later becomes Hush) reveals that an associate of his had riddled him to the mystery of Batman's secret identity. It still remains unclear how or when Nygma discovered that Bruce Wayne was Batman.
Poison Ivy
When it comes to Pamela Isley, the iconic Gotham villainess has actually been called out on two Earths in the Arrowverse. The first time is throughout the Elseworlds event where a cell belonging to "P. Isley" is spotted, confirming that DC's eco-terrorist exists on Earth-1. Nothing else has been revealed about Ivy on this Earth ever since that Easter egg was shown. While her Earth-38 doppelganger is never seen either, Pamela is referenced in Supergirl season 4 in Lex Luthor's journal. In that world, Pamela is established as Poison Ivy whose pheromone control was something Lex became interested in.
Bane
Even though his name is never said, Bane's mask can be seen on a shelf in Arkham Asylum. What is interesting, though, is that rather than his traditional luchador mask, it's actually the same helmet as Tom Hardy's version of the Batman foe from The Dark Knight Rises. It's unlikely that the Earth-1 version is actually a doppelganger of Hardy's incarnation of Bane, and this more probably just a nod towards Christopher Nolan's Batman trilogy. Despite being supposedly locked up at Arkham, it's unclear whether or not the Knightfall saga has happened in the Arrowverse before Bruce disappeared from Gotham.
Scarecrow
One of the big sequences in Elseworlds is when Oliver Queen and Barry Allen begin fighting their respective nemeses, as Malcolm Merlyn and Reverse-Flash emerge in front of them. What had actually happened was that they had been infected by Scarecrow's fear toxin, causing them to both see their worst fears. Batwoman ends up saving Green Arrow and The Flash from each other as she discovers that they had been affected by Jonathan Crane's gas. His fear toxin also ends up coming into play in Batwoman season 1 as both Mouse and Alice are poisoned by Scarecrow's gas.
Clayface
There have been multiple people in the comics who go by the Clayface name, and while none of them have physically appeared, one of them is established through a name-drop. Another name that is seen during the visit to Arkham Asylum in Elseworlds is "B. Karlo," which references the first Clayface. Even though they never see him, viewers can see a handprint on the cell window as a nod to his abilities. During Crisis on Infinite Earths, Bruce Wayne of Earth-99 references the Clayface of that world when dismissing theories on how Kate can be standing in front of him, given that her doppelganger died on his Earth years ago.
The Penguin
Another major villain from the Batman lore that is established in the Arrowverse in name only is Oswald Cobblepot a.k.a. the Penguin. Like the majority of Bruce's enemies, Cobblepot's name can be seen on one of the cells in Arkham Asylum. However, Batwoman season 1 ended up confirming a major event regarding this world's Penguin: before he got locked up, Cobblepot was mayor of Gotham City at one point. Since that reference, the Penguin's status in the Arrowverse hasn't been clarified as he is presumably still at Arkham.
Mad Hatter
Despite Alice being a primary threat in Batwoman, the Mad Hatter has yet to be seen, at least on Earth-Prime. However, prior to Crisis on Infinite Earths changing the Arrowverse forever, The Flash season 5 ends up establishing Jervis Tetch existing on another Earth. Referred initially as the "Terrible Mad Hatter Killer," Sherloque Wells tells the team about the serial killer from Earth-221 and how his partner tried to stop Tetch. Even though he has yet to be referenced on Earth-Prime, by existing on Earth-221, it confirms more or less that he has doppelgangers out in the Multiverse, including the new Earth that the Spectre created.
The Joker
Without a doubt, the Joker is Batman's ultimate villain. While the Arrowverse confirms the existence of the iconic clown prince of crime, the Joker's status came with a bit of a twist. After several references in Batwoman season 1, including his real name being Jack Napier, Luke Fox ends up telling Kate that there is a reason why he hasn't been spotted in years. For unknown reasons that have yet to be clarified, Bruce actually killed Joker over 5 years ago. Despite this twist, it wouldn't be a major surprise if the Joker at some point re-emerges in the Arrowverse. When it comes to the Dark Knight's greatest nemesis, anything is possible.
Killer Croc
One of Batman's more monstrous foes is Killer Croc, who actually ends up getting called out early on in the first season of Batwoman. While the name-drop came in the form of a comedic line, it at least establishes that he is somewhere out there in Gotham City and has clashed with Batman. It wouldn't be impossible to see Croc at some point, given that The Flash has been able to do CGI-heavy villains like Gorilla Grodd, King Shark, and Solovar. Even Legends of Tomorrow has featured fan-favorite Beebo multiple times, who is also a CGI-heavy character. It certainly wouldn't be impossible for Batwoman to maybe one day bring in Croc for an episode or two.
Jane Doe
Another identity-challenging enemy that Batman has gone up against is Jane Doe, who ends up being referenced during Crisis on Infinite Earths. While the crossover was spending time on Earth-99, it established the identity-stealing villain existing in the Arrowverse. This could pave the way for the Earth-Prime version of Jane Doe to show up in Batwoman season 2 or in later seasons. With these shows, sometimes name-drops end up leading to those characters actually popping up on the respective shows. It would also be a solid way for the series to explore one of the smaller Batman villains.
Two-Face
Another iconic Batman villain that has been teased in the Arrowverse is Two-Face, but with a twist. While most of the Caped Crusader's enemies have been active for years, Harvey Dent seems to be one of them that has yet to become his villainous comic counterpart. During the second half of Batwoman season 1, the Joker's daughter was introduced as the villain-of-the-week in episode 14. Through her guest-spot, it's established that she is Duela Dent, Harvey's niece, as he is referenced as being a beloved ADA. This reveals that Harvey has yet to go down his villainous path, which could tease the idea of future Batwoman seasons doing the origin story of how he becomes Two-Face.
Azrael
The latest Batman villain to enter the Arrowverse recently was a surprising one, as it possibly confirms that the Knightfall storyline has already taken place. In one of the final episodes of Batwoman season 1, Azrael - who has both been a foe and ally to Batman in the lore - is referenced along with the Order of St. Dumas and the Order of Purity. In episode 19, "A Secret Kept from All the Rest," Azreal's name is seen in Lucius Fox's journal as it focuses on the Suit of Sorrows. While it's likely that the Azrael they're referring to is Jean-Paul Valley, the first person to carry the title, it could very well also be Michael Lane or Abraham Arlington.
With Batwoman season 2 arriving in January 2021, only time will tell what other iconic Batman villains will get referenced or even introduced. As the series moves into a new direction with Leslie's Ryan Wilder taking over the cape and cowl from Kate, anything is possible for how much deeper they will explore the Batman mythology. But given that they have already teased so many iconic villains already, it would be better for the Arrowverse to start paying off those references by introducing the respective Batman foes that they have established already.