There are so many other ing characters from the comics who may never quite get their own line or even narrative arc but could fit into Peacemaker's own world really well. They might have links to him in the source material, have previously been mentioned on the show, or possess a gimmick that would contrast well with the lead vigilantes.
Kite-Man
Peacemaker has already made brief mention of Kite-Man and the character truly is comedy gold. He's one of the funniest of team Harley Quinn in the animated show and has a quirky personality to match his ridiculous premise. Only James Gunn could really bring this highlight reel into live-action.
The kite itself is a fun idea to play around with and there is surely a range of action sequences that could be created using Kite-Man's famous devices. Of course, Peacemaker is hardly going to take him seriously, but a Vigilante and Kite-Man team-up has so much potential.
Catman
There are so many alternative Catwoman variants that could be adapted (and indeed have been), but there have been no iterations of Catman yet. The ridiculous character is a Batman villain and carries off a similar premise to his namesake, becoming a competent jewel thief.
The parallels between the two characters are absolutely something that Peacemaker could make fun of, and considering he has a problem with Batman, he might take issue with the fact that Catman's costume is very much inspired by the Caped Crusader. Even though Catman has the potential to be a comedy character, there have been some deep arcs featuring the crook that may play well in Peacemaker.
Condiment King
Condiment King is one of the weirdest designs DC has put together and has always been used more as a gag than any kind of serious antagonist. The reason for this is that he fires all kinds of weaponized condiments, from ketchup to mustard, which can end up having a lethal effect on a person if they are hit. He too would fit into Peacemaker's vibe well.
There is no shortage of opportunities for whimsical situations to be created using Condiment King. The DCEU has housed all manner of oddities so this is surely a villain that exists already and perhaps one brought into Task Force X (if any part of it is left).
Zatanna
It might seem like a strange change of pace but, so far, Peacemaker has dealt with otherworldly threats from the stars. For the second season, it might be compelling to see the antagonists shift in of where in the DC Universe they originate from.
Zatanna has rarely been seen on screen, but for some kind of supernatural situation, she's the perfect leader for Peacemaker to follow into a fight. Their dynamic would be a tense one as Zatanna wouldn't stand for his nonsense and the powerful DC magic wielder would certainly bring something unique to the table (narratively speaking).
The Human Target
The Human Target has even had his own show previously and has been spotted in the Arrowverse before. However, he would be ideal as a member of the ARGUS team in Peacemaker, with his ability to change his appearance becoming incredibly useful in the field.
The Human Target is an operative with plenty of experience surviving dangerous situations. The character would provide a lot of incredible twists and what's more, might actually bounce off of Peacemaker well (perhaps as the straight-man without all the theatrics).
Matter-Eater Lad
Strangely enough, Matter-Eater Lad was one of the characters Peacekeeper namedropped within the series, stating that he'd previously worked alongside the oddity. His ability really is to eat anything and everything, which could be quite useful in a bizarre way.
With a tease like that, season 2 has to follow up on it. It's the kind of character that Vigilante would think was awesome and since there's a shared history with Peacemaker, there's something to explore there too.
Killer Frost II
While Killer Frost has become a mainstay of the Arrowverse, the source material actually sees a new character take on the name. Louise Lincoln is the second iteration of Killer Frost and very much lives up to the deadly name.
The character hasn't been used to her full capabilities yet. Indeed, she's mostly known for being featured in animation, but James Gunn could put his own spin on the character. Peacemaker interacting with powered individuals would change the dynamic of the ARGUS squad, while Lincoln is the kind of erratic force that could shift everything. Perhaps Peacemaker would even feel slightly fatherly towards the rogue.
Tattooed Man
James Gunn loves to include random side characters with no real connection to the larger plot but still link back to the comics. Calendar Man is a perfect example of that in The Suicide Squad, but for Peacemaker, the Tattooed Man might fit the role well.
He's just a guy with a tonne of tattoos, although, in some variations, he has been able to bring the images to life. That's the kind of out-there concept that fits into Peacemaker well, especially if the prison becomes a focal point of season 2 like it did with the first installment.
Spoiler
The Bat-Family boasts a huge range of very competent combatants and Spoiler is amongst their toughest. She has a strong history of vigilantism in the comics already and is the kind of street-level hero that would fit well into the show.
She could be assigned to the group to keep them all in line, especially given some of the murderous antics Vigilante gets involved in. Considering Peacemaker hates Batman, coming across one of his prodigies is sure to rub him up the wrong way, especially as she is far more capable than him.
Unknown Soldier
Unknown Soldier is very much a man of war and has been portrayed in a variety of ways. From a patriot to someone who's disillusioned with the system, regardless of the variation the political narratives associated with him really play well into Peacemaker.
As a fellow soldier, Peacemaker would have a lot of respect for him, but their methods may massively differ. Having a true secret identity on the team as well could cause trust issues, which the screenplay could use to create tense conflicts. In of his skillsets, visually Unknown Soldier and his variety of weapons match the tone of the series.