It's been many years since Batman: Arkham Knight was released. The end to developer Rocksteady's successful trilogy that reinvented, not only Batman but licensed videogames in general. While 2021 will see Gotham Knights, it will still be another year until Rocksteady's next title.
That next game set in the "Arkhamverse" is Suicide Squad: Kill The Justice League, a team-based action-adventure-shooter coming in 2022. It already features the core playable villains of Deadshot, King Shark, Captain Boomerang, and Harley Quinn, with Superman villain Brainiac being an antagonist. However, that's not to say there isn't room for more. There are still plenty of DC villains who should be in the game, either playable or corrupting the minds of the Justice League.
Deathstroke
When the New 52 re-launched the Suicide Squad comic as "The New Suicide Squad," it picked Slade Wilson's Deathstroke as the leader, where he had a fantastic rivalry with the similar masked-assassin-for-hire Deadshot.
While Rocksteady would have to take careful skill not to make Deathstroke play too much like Deadshot in the game, there is precedent for his involvement. The end of 2011's Batman: Arkham Origins teased a Suicide Squad videogame, with Deathstroke planned as the leader of the group. While the game's changed developers since then, Deathstroke is too popular and too perfect a fit into the game to not be involved.
Bloodsport
Robert DuBois is an expert in firearms, consumed by grief over his brother being killed in warfare after he, himself, backed out of military service. He ended up gaining the of Lex Luthor and took on Superman with Kryptonite bullets.
While he did make an appearance on Supergirl season 3, in 2021, Bloodsport will be seen in The Suicide Squad movie, where he will be played by Idris Elba. With such a prominent part in the movie, it seems likely Bloodsport will appear in the Suicide Squad videogame too.
Enchantress
June Moone is host to the ancient godlike witch known as the Enchantress, who alternates between hero and villain. She also flips between being a regular magical Suicide Squad member and becoming a threat the Squad has to deal with, as seen in 2016's Suicide Squad movie.
Enchantress's magic abilities would make for a unique playable character in Kill The Justice League. Alternatively, she could also be one of the major antagonists, as she could be controlling the mind of a member of the Justice League or sending her eyeball-creatures out to fight.
Killer Frost
Caitlin Snow, otherwise known as Killer Frost, has been on the Suicide Squad more than once, including in the recent "DC Rebirth" comic series. She also appeared on the team in the Batman: Assault On Arkham animated movie - the Suicide Squad movie that is set in the same "Arkhamverse" continuity as Suicide Squad: Kill The Justice League. She's also a regular on The Flash TV series, where she's played by Danielle Panabaker, so she's a well-known character who would be perfect to play in the game.
Despero
The alien supervillain, known as Despero, is one of the most dangerous adversaries the Justice League has ever faced. He has the strength to stand up to Superman but also has a third eye that grants him psychic and psionic powers.
These abilities alone would make him a formidable adversary for the Suicide Squad, but it is Despero's power to hypnotize that would make him a good choice for Suicide Squad: Kill The Justice League. It is entirely possible that Despero has mind-controlled a member of the Justice League, so the Squad would have to take both of them out to succeed.
Peacemaker
Christopher Smith a.k.a. Peacemaker was originally a hero, but his mantra of peace at literally any cost, no matter how many lives he has to take to achieve it, was at odds with the Justice League and Earth's other heroes.
While more of a deadly vigilante than a villain, Peacemaker will be a major part of the The Suicide Squad, where he will be played by John Cena. In fact, Peacemaker seems to have a bright future ahead for him since he's become the first DCEU character to receive their own HBO Max series.
With Peacemaker's popularity presumably about to skyrocket, Rocksteady would do well to include him in the Suicide Squad game, and with his rocket pack and a vast array of weaponry, he would fit in well.
Starro The Conqueror
There is no reason to assume that Brainiac will be the only one controlling the minds of the Justice League. All the Batman Arkham games had multiple threats, so Suicide Squad could too. If so, there would be no antagonist better suited for the story than the psychic starfish Starro the Conqueror.
Starro was the first enemy the united Justice League ever faced and can completely take over the minds of any being, so he could be controlling at least some of the League. Starro made a brief Easter egg appearance in Batman: Arkham Knight's DLC and Rocksteady have previously used such secret areas to tease their next game. Plus, Starro is also rumored to be the big bad of The Suicide Squad movie.
El Diablo
The most likely villains to appear as playable in a Suicide Squad game are the ones with unique power-sets that are well-known to fans, and Chato Santana's fire-spewing El Diablo certainly counts as both.
While he's known for his pyrokinetic powers rather than firearm use, as a former ruthless gang leader he's fully capable of using guns, and the game could combine regular weapons with his fire powers to devastating effect.
The Trickster
While Suicide Squad: Kill The Justice League does feature the Joker's ex-girlfriend Harley Quinn, the game (and the Squad itself) would benefit from having an even more chaotic villain on the team. As the Joker is canonically dead in this universe, the next best choice would be the Trickster.
In some ways, he would be better. In fact, as James Jesse is often shown with more amusing gadgets and even hover-boots, all of these elements would make him fun to play. Furthermore, after being portrayed by Mark Hamill on The Flash as a creepy mix of Joker and Hannibal Lecter, fans are eager to see the Trickster return.
Cheetah
With her recent starring role as the main villain of Wonder Woman 1984, Barbara Minerva's Cheetah really should make an appearance as a playable part of Suicide Squad: Kill The Justice League. Her godlike agility would allow players to bound across the Metropolis map quickly.
She would be a more melee-focused character for sure, which might feel odd in a shooter, but her lightning reflexes and powerful claw attacks would easily shred enemies. It's entirely possible that Cheetah is coming back for Wonder Woman 3, so being playable in the Suicide Squad game between her two movie appearances would be perfect.