No matter how many minor characters break free from the comic book page and make it to the big or small screen, there will still be a seemingly endless supply to draw from. Thankfully, James Gunn is more than happy to work through the list, one project at a time.
Peacemaker is one of the most popular superhero properties of the modern era, despite working with a cast of D-listers and below. Gunn clearly loves minor comic book mainstays, from his Guardians of the Galaxy trilogy to The Suicide Squad, he packs his cast with lesser-known names. One of the most fascinating aspects of the series is that Gunn is not afraid to experiment with the characters to make them fit in. His takes on White Dragon or Vigilante made the characters far more engaging and brought new love to largely unknown characters. The series' big villain, the butterflies, was an entirely original invention of Gunn's. He could do the same for the upcoming season, but countless other villains could pop up in one role or another.
Black Spider
The best possible resource for this question comes from a wonderful James Gunn Twitter thread wherein he shows off characters he considered dropping into The Suicide Squad. Black Spider made the list, and Peacemaker seems like a great place for him to make his debut. Though the fantastic series Young Justice turns him into something akin to a DC Spider-Man, his original iteration is a vigilante and assassin.
Eric Needham became addicted to heroin in his youth and killed his own father in pursuit of cash to fuel his drug problem. He kicked the habit in later life and set to work brutally murdering drug dealers and suppliers. Batman's aversion to murder led him to intervene and arrest Needham, sending the vigilante onto the path of costumed villainy.
There is actually a very interesting story here about a man who keeps trying and failing to do good after struggling with addiction and guilt. Some elements even neatly pair Peacemaker's personal struggles. Black Spider unfortunately never gets the time necessary to explore that narrative, but if anyone can fix that, it's James Gunn.
Killer Moth
From an arachnid to an insect. Killer Moth is probably best remembered from a single episode of Teen Titans, but the comic book character isn't similar to that iteration. Drury Walker is an ex-convict who defined himself by his opposition to Batman.
Like the Caped Crusader, Walker lacks superpowers and gets by with tech and gadgets. The most interesting thing about Walker is his dedication to helping other criminals get away with their misdeeds. Killer Moth is the founder of a team called The Misfits, which is a league of minor DC villains who've banded together to fight for relevance and profit. Calendar Man, Catman, and Chancer make up the ranks of the team. The idea of a team of minor villains working together to do battle with Peacemaker and his team of government agents is a hilarious and engaging hook for season 2.
Chemo
Some characters are just what they say on the tin. This is another of the list James Gunn mentioned for The Suicide Squad, but this villain is a unique threat. Chemo is a barely sentient living mass of experimental chemicals which mindlessly destroys whatever enters its path.
When Chemo is around, villains, heroes, and intermediaries set aside their differences and set to work trying to dispose of it. Chemo can't really be killed, as it isn't alive in any traditional sense. This may seem like a threat out outside of Peacemaker's capabilities, but so was the Butterfly invasion of season one. Chemo is a great ticking clock-style season-ending threat and James Gunn could certainly find some fun things to do with a walking blob of radiation.
Gorilla Grodd
Gunn and Steve Agee sure seemed to get a lot of fun out of butchering an ape in season one, it would be interesting to flip that joke on his head with a more powerful gorilla. Loads of fans speculated whether Charlie, the gorilla who was taken over and killed, was setting up for Grodd, but those guesses were inaccurate. Grodd is a hyper-intelligent ape with psychic powers. He's typically a big villain in the DC universe, often employing armies of henchmen and operating long-term plans.
Grodd finds a perfect sweet spot amongst DC villains, strong enough to seem threatening, yet not so strong that Peacemaker and company could never reasonably defeat him. Grodd would make a fascinating final foe for the second season, and his army of apes and psychic powers make him capable of summoning up an enemy army.
Knockout
Yet another pull from Gunn's Twitter thread. Peacemaker's relationship with women, aliens, and women taken over by aliens has been a common topic of conversation for the series. Though the season ended with some evidence that Smith and Harcourt might get together after all, a superhuman stripper could shake that relationship up.
Knockout is a member of the New Gods and a warrior previously in service to Darkseid who lives on Earth. She was forced to aid the Suicide Squad, she's fought Superboy, she joined the Secret Six as a mole. Knockout is typically in a relationship with half-immortal assassin Scandal Savage, and the pair typically come as a package deal. Both are extremely strong and possess inhuman healing factors, making them powerful foes. Knockout and Scandal are a power couple who would be dynamic set against the cast of Peacemaker.