While many superhero series employ the villain-of-the-week format, Batwoman mostly sticks to villains with long arcs instead. So far the series has seen the titular hero take on comic book villains Alice, Hush, Safiyah, and Black Mask. Some of those villains have more staying power than others. Alice, for example, is likely never leaving this version of Gotham since her story is so entwined with both women who have become Batwoman.
The series wouldn't be remiss to include a few more villains-of-the-week as it moves forward to give audiences a breather from longer arcs. It also might be fun for fans to see villains brought to life that have already been mentioned in the series, but have yet to make an on-screen appearance. Batwoman is at a slight disadvantage compared to CW series like The Flash in that her comic book rogues gallery isn't quite as deep. The series, however, takes place in a world where many iconic Batman villains are already established, leaving space for lesser-known Batman and Batwoman villains to take center stage if the writers so choose.
The Mad Hatter
The Mad Hatter seems like a no-brainer for a show like Batwoman. Alice and the Mad Hatter, Jervis Tetch, both take their inspiration from Alice In Wonderland. The two villains, however, have no real connection to one another. In fact, the Wonderland Gang of the comics belongs to him, not Alice, though she employs it in Batwoman's first season. He's long appeared on villain wishlists for the show.
With the series taking Alice on a potential journey of redemption though, his presence in the Arrowverse becomes more welcome. If Alice is truly able to become an antihero as she does in the comics, someone else using her schtick might not sit well with her. It would definitely be fun to see the two battle it out with Alice quotes while literally battling it out on the streets of Gotham. A version of Tetch also appeared in FOX's Gotham. The series is far enough removed that most fans likely won't be spending a lot of time comparing the two.
Weeping Woman
The Weeping Woman comes to the comics at a time when Batwoman's arc is focused on urban legends - and Greek mythology. Those two concepts fuse in the criminal organization MEDUSA, named after the Greek Gorgon. One of the villains brought to life for the organization is the Weeping Woman, inspired by the urban legend La Llorona.
Maria Salvaje, a single mother who loses her children is targeted by Maro Ito, who can actually transform people into the subjects of urban legends. Her story would make for a creepy episode as he transforms her into a ghost-like figure made of water. She also doesn't want to be a villain but is forced into the position. Eventually, she turns on him and breaks her own "curse" herself. Batwoman has no shortage of women fighting for themselves and the city, but Maria would give the audience that same idea from the villain's side.
Green Fairy
Green Fairy only appears in a handful of comic books with very little known about the character. That makes her a character the show could literally do anything with. Her inclusion in the series could also serve to expand the kind of power sets seen in the show since most of Batwoman's villains aren't metahumans, unlike other Arrowverse shows.
Her few appearances in the comics reveal her as an inmate of Arkham Asylum. She escapes when the facility is broken into. With green skin and wings, she bears a slight resemblance to DC's character Titania as well. In addition to flying, she exhales toxic gas that can scramble her opponent's faculties, making it easier to win a fight. Even her real name hasn't been revealed in the comics, leaving her backstory a big mystery for fans.
Killer Croc
This particular choice might seem like an odd one. After all, Killer Croc was recently on the big screen in Suicide Squad. Including him in Batwoman would also likely take painstaking makeup work or impressive CGI. That doesn't mean he should be written out completely.
Technically, the character already exists in the Arrowverse, but the audience has never seen him. Kate Kane makes a joke about him in the first season to her girlfriend at the time. He's never mentioned again. With Ryan Wilder taking over the reins as Batwoman, however, the show has room to explore a lot of social issues in Gotham that Kate Kane's tenure didn't. The series has already brought police brutality, for-profit prisons, and homelessness to the front of the storylines in the second season. In most versions of his story, Waylon Jones becomes a criminal because he's ostracized by society for his appearance. Ryan would certainly sympathize with that. She might even try to find a way to actually help him instead of simply fighting him.
Poison Ivy
These days, Poison Ivy is most often associated with Harley Quinn. She got her start, however, as a Batman villain. Many adaptations of the character have made it to the screen. Fans are divided on whether they love them or hate them. While she's initially a femme fatale, modern stories have her as an environmentalist, and sometimes ecoterrorist.
Poison Ivy might be violent and out to get humanity on a regular basis, but her points are solid. She thinks humans are destroying the planet with their lack of care. All she really wants is for plant life to stop being destroyed. That's not such a hard thing to understand. She also has a soft spot for kids, never intentionally putting children in harm's way, much like Harley Quinn. Her own code of honor - and her desire to actually help the planet - would mean Team Batwoman might not be in such a hurry to take her out. Batwoman and Poison Ivy might even be able to make an agreement that would allow them to coexist on the streets of Gotham.