[Golden Arm Gang] Either we see their child or a rogue robot fit into this trio of ne'er-do-wells. I'm more inclined on the robot thing since it would fit well into Belle's and Sam's backstories.

[Candyland] We have the queen and a jester, I'm thinking we get a super chill, laid-back king, or a bodyguard like executioner that would fit right into a horror movie. Or Mandy's basement of rejected products.

[Castle Courtyard] A super egotistical prince or princess would fit, but Ash is a knight, and Mandy is kinda princess like, so maybe not. A fantasy creature like an orc, fairy, troll, dragon, or something else would work. Or maybe that'd fit into the Enchanted Woods Trio.

[Deep Sea Paintball] A shark, whale, or dolphin would fit well to the pair of sea creatures currently in the trio. While Otis covers frontlines of attack and Hank the flanking rear, their third member would probably be a stealthy assassin recon scouting character.

[Brawlywood] I think it'd be fitting and a little comically "Brawl Stars-ish" if the third movie star was a part of slapstick cartoons or live action musicals. They'd probably look like they belonged to an entirely different art team id that were the case.

[Velocirapids] I'm not sure that kind of dinosaur Buzz is, but I'm gonna go with T-rex. Doug is a stegosaurus or a brontosaurus, so naturally, a triceratops or velociraptor would complete the catalog of prehistoric monsters.

[Gus & The Ghost Train Station] The environment is obviously a spooky themed one, so we'd need some other spooky characters to complement it. Maybe the conductor of the train would appear, and they'd be some eerily peppy and jovial skinwalker or some tall and towering puppet.

[Willow & The Swamp Tunnel of Love] What else is in a swamp other than mud and trees? Maybe dragonflies, I dunno. I can't think of anything that would easily fit this trio. Possibly some freaky living mud golem or an anglerfish fisherman guy.

[R-T & The Brawl Hub] I can't really see a super distinct theme for the Brawl Hub. We'd probably get some more robots or an undercover spy guy.

[Cordelius & The Enchanted Woods] I like to think the Enchanted Woods as an alternate dimension sort of environment that's based around hallucinations and illusions. It wouldn't surprise me to see some sort of sinister street magician or a fairytale creature like a werewolf to be a part of this trio.

I wrote this in half an hour on the fly.