In the reveal trailer for Nickelodeon All-Star Brawl 2, a set of gameplay footage from the first installment is shown. This is notable for two reasons, one being that the footage shows off multiple characters not featured in the second game, and that one scene shows a beta version of the Wild Waterfall stage with a smaller waterfall and different colors for the Thornberrys' ComVee. This would lead to a theory that the Wild Waterfall footage was taken from NASB 2 and accidentally inserted into the NASB 1 section of the trailer, which if true would confirm that Lincoln Loud and Leonardo were returning.

This theory was quite easily debunked by Lincoln and Leonardo using their models from the first game while everyone else got revised models, and a KO explosion effect featured in the snippet being different from one shown later in the trailer. Nigel Thornberry's character showcase would later show off a new Wild Thornberrys stage in place of Wild Waterfall, which was shortly followed by a leak showing Lincoln as an NPC cameo on the Loud Castle stage.

Nickelodeon originally wanted the developers of Nickelodeon All-Star Brawl to use the 2012 versions of the Teenage Mutant Ninja Turtles. However, they pushed to use the 1987 versions instead despite that iteration never having aired on Nickelodeon at the time, with their reasoning being "we're old".

Following the success of All-Star Brawl, other Nickelodeon crossover games would begin using the 1987 TMNT, including Nickelodeon Kart Racers 3, of which the previous two games used the 2012 TMNT, and Nickelodeon would begin airing the 1987 TMNT animated series on their channel in promotion for the film Teenage Mutant Ninja Turtles: Mutant Mayhem.

When asked about why the development team behind Nickelodeon All-Star Brawl chose to include Powdered Toast Man from "The Ren & Stimpy Show" as a fighter, a character from a 30-year-old series who only appeared in 6 out of 52 episodes, Ludosity CEO Joel Nyström responded: "He’s fun. Videogames are fun. It’s a great match."

When SpongeBob is on-stage in Nickelodeon All-Star Brawl, he automatically activates a code titled "yummerSummon" - this does absolutely nothing, and serves as a comical reference to an infamous unused graphic in SpongeBob Saves the Day! for dataminers to discover. The Yummer graphic is not known to exist in All-Star Brawl.

Despite MultiVersus being positioned by the gaming press and certain players disappointed with the latter game's lack of content as being a "rival" for having "killed" Nickelodeon All-Star Brawl, the development teams behind both games are fans of one another's work.

In an interview with competitive Super Smash Bros. player Hungrybox, the developers of Nickelodeon All-Star Brawl claimed that the game would feature cross-play in a future update. Cross-play was eventually finished for the game, but was not able to be released due to Nintendo's strict guidelines on cross-play. On March 3, 2023, the game was released on the Epic Games Store, and cross-play between PC and Switch was accidentally added without announcement, but was removed shortly after, though cross-play between Epic Games and Steam players would continue to exist in the game. Cross-play would eventually be included in the base game of Nickelodeon All-Star Brawl 2 for all consoles and generations.

Nickelodeon All-Star Brawl contains multiple unused announcer lines including:

• Several character names (including characters from Rocko's Modern Life and The Adventures of Jimmy Neutron: Boy Genius, which were not represented in the base game)
• Item announcement calls which did not end up being used (despite the narrator having used voice clips relating to the items)
• All of the stage names (notably including alternate names for the Slime Time stage in the form of "Double Dare", "Double Dare Stage", and "Double Dare Extreme")
• Perhaps most interestingly, lines relating to a shop feature where the player could buy outfits, characters, moves, and stages. The reveal press release for All-Star Brawl references the ability to "unlock advanced moves", which may be referring to this scrapped feature.

Upon the release of the Hugh Neutron DLC, a glitch was added to the Switch version of the game that made Aang's air ball move an instant KO. This was quickly patched out.

All but one of the competitive stages in Nickelodeon All-Star Brawl are inspired by stages from the Super Smash Bros. series:

• Jellyfish Fields from SpongeBob SquarePants is based on Smashville, introduced in Super Smash Bros. Brawl.

• Harmonic Convergence from The Legend of Korra, and the post-update version of Royal Woods Cemetery from The Loud House, are based on Final Destination, introduced to the multiplayer stage lineup in Super Smash Bros. Melee.

• Sweet Dreams, from Garfield, is based on the frozen version of Super Smash Bros. Melee's Pokémon Stadium, introduced officially in Super Smash Bros. Ultimate, but popularized by fan projects such as the Super Smash Bros. Brawl mod Project M prior.

• Irken Armada Invasion, from Invader Zim, is based on Battlefield, introduced to the multiplayer stage lineup in Super Smash Bros. Melee.

• Rooftop Rumble and Sewers Slam, both from Teenage Mutant Ninja Turtles, are based on the hazards-off version of Smashville introduced in Super Smash Bros. Ultimate, with Sewers Slam - originally intended as a casual stage but adopted into the competitive stage lineup - containing a goop hazard somewhat resembling the wind from Super Smash Bros.'s Dream Land.

• Duck, Duck, Pie!, based on The Adventures of Jimmy Neutron: Boy Genius is based on the hazards-off version of Super Smash Bros. for Wii U's Kalos Pokémon League, introduced in Super Smash Bros. Ultimate.

• The exception to this pattern is Tremorton Joyride from My Life as a Teenage Robot, which is based loosely on Delfino's Secret from the Super Smash Bros. Brawl mod Project M.

Unusually, Rocko's light up air (where he puts on boxing gloves and deals a flurry of punches) is more powerful than his strong up air (where he forces his opponent to look at a painting of a sad crying clown in an iron lung), with the prior move dealing 19% maximum (14% coming from the last hit alone), while the latter move only deals 13%.

Nickelodeon All-Star Brawl has certain side characters (Helga, Nigel, Hugh, Reptar, Oblina) represent their respective series as playable characters. The main protagonists of these series (Arnold, Eliza, Jimmy, Tommy Pickles, Ickis) are absent from the game in any physical form, with only Tommy and Arnold making brief cameos as photographs in props on the Teeter-Totter Gulch stage, and one of Helga's attack props respectively.

Also of note are Raphael and Donatello, one half of the Teenage Mutant Ninja Turtles who are not playable in the game despite their equal importance to Leonardo and Michelangelo, with Shredder and April taking their place. They do feature as a profile icon, but are absent from the Teenage Mutant Ninja Turtles stages.

Many Nickelodeon fans noticed a significant omission of characters from the series The Fairly OddParents in Nickelodeon crossovers starting in 2008's SpongeBob SquarePants featuring Nicktoons: Globs of Doom, which was the final installment in the Nicktoons Unite! series where Fairly OddParents protagonist Timmy Turner had previously been part of the main ensemble cast. The reasoning for this is believed to be due to Nelvana, a Canadian distributor of children's content, holding the international licensing rights and copyright for the first 5 seasons of the show outside of the US, having used this to syndicate the cartoon on the rivaling Disney Channel.

Timmy Turner did later appear in the 2022 Apple Arcade game Nickelodeon Extreme Tennis. In response to a tweet speculating that this could lead to Fairly OddParents content appearing in Nickelodeon All-Star Brawl, the community manager for the latter game Thaddeus Crews posted a "heads-in-the-clouds" emoji. A playtester for Nickelodeon All-Star Brawl 2 alleged that the legal issues with The Fairly OddParents are so complicated and strung-out that a former Nickelodeon or Nelvana employee was gifted the copyright to Timmy Turner's pink hat design as a farewell gift.

In Garfield's showcase presentation, the narrator remarks "It's time to kick Odie off the table!" when showing Garfield's light neutral attack. This is a reference to "Garfielf", a surreal fan animation by YouTube animator PilotRedSun. While kicking Odie off the table is a recurring gag in Garfield comics, the specific quote "It's time to kick Odie off the table" originates from the Garfielf video and seems to not be from any official Garfield media.

The layout of the Traffic Jam stage in Nickelodeon All-Star Brawl, consisting of a bus driving along moving asphalt that players can fall onto and either ram into the bus or get conveyed off-screen, while cars appear to the left and right to stand on, is recycled from a stage from Ludosity's first platform fighter Slap City called Public Transit, with the main difference aside from the Hey Arnold! theming being the absence of an equivalent to the Apathetic Frog helicopter.

The background music for the Slap City stage is also called "Traffic Jam" on the game's song selector (though the official Bandcamp release and the internal file names call the song "Noisy City" instead) and a side-scrolling stage reusing assets from Public Transit in Slap City's story mode is called "Traffic Jam".

Two of the franchises represented in Nickelodeon All-Star Brawl have had installments aimed exclusively at adults. Both of these franchises have their mature iterations referenced within the game, with Ren & Stimpy's taunt originating from the Ren & Stimpy Adult Party Cartoon episode "Fire Dogs 2", and Leonardo and Michelangelo's alternate costumes originating from the original Mirage Teenage Mutant Ninja Turtles comics.

When Garfield was released as a playable fighter in Nickelodeon All-Star Brawl, Twitter user InsaneProtayto posted a viral tweet with a fake screenshot of an IGN news article claiming the cat couldn't be played as on Mondays, referencing one of his most well known catchphrases, "I hate mondays". This hoax ended up becoming so popular that publisher GameMill Entertainment responded on Twitter confirming that Garfield can in fact be played as on all days of the week.

In Sports mode, the Arnold football has an indicator above it to inform players that it must be grabbed and not hit. This graphic, however, is a crude placeholder graphic that appears to have slipped into the final game.

Every character in Nickelodeon All-Star Brawl has an internal codename used instead of their name. These are:

• Orb - Garfield (Garfield is fat)
• Moon - Leonardo (Unknown, but possibly a reference to the opening shot of the '87 TMNT cartoon's theme song being the moon)
• Pizza - Michelangelo (The Ninja Turtles are known to love pizza, with Michelangelo being the biggest fan of the food among his brothers)
• Cheese - Shredder (Shredder's character design was based on a cheese grater)
• Reporter - April (April is a news reporter)
• Mascot - Reptar (Reptar is a fictional character within the Rugrats universe who appears on a variety of products, particularly toys and food)
• Duo - Ren & Stimpy (Ren & Stimpy are two characters who fight as one unit)
• Hero - Powdered Toast Man (Powdered Toast Man is a parody of superheroes)
• Squirrel - Rocko (A running gag in Rocko's Modern Life involves Rocko being mistaken for other animals, such as kangaroos, beavers, and squirrels, instead of a wallaby)
• Snake - Oblina (Oblina has a thin, somewhat curved shape akin to a snake)
• Rival - Helga (Helga has a love-hate relationship with series protagonist Arnold)
• Narrator - Nigel (Nigel hosts his own nature show-within-a-show called Nigel Thornberry's Animal World)
• Chimera - CatDog (CatDog is two animals with a single body)
• Apple - SpongeBob (SpongeBob lives in a pineapple)
• Star - Patrick (Patrick is a starfish)
• Diver - Sandy Cheeks (Sandy is a land mammal who lives underwater)
• Alien - Zim (Zim is an alien invader)
• Pie - Hugh (Hugh loves pie, and his home stage is a giant pie)
• Neo - Jenny (Jenny is a futuristic robot, and quite young, being a teenager mentally and having been built 3 years ago in the show)
• Plasma - Danny Phantom (Danny's main method of attack is shooting ghost plasma rays.)
• Kite - Aang (Aang travels using a glider, which he uses as an attack in NASB)
• Clay - Toph (Toph forms clay structures out of ground)
• Athlete - Korra (Korra partakes in a sport known as Pro-Bending)
• Rascal - Lincoln Loud (Lincoln is a mischievous child)
• Goth - Lucy Loud (Lucy is a goth)

The inclusion of Hugh Neutron in the game as DLC, particularly instead of his son and series main protagonist Jimmy Neutron, was a response to an ironic internet campaign to get the character in the game known as "Hugh Nation", led by Hugh voice actor Mark DeCarlo and YouTubers The Superverse and Uncle Al, following Hugh trending on Twitter due to memes related to the game's reveal. When Hugh performs his strafe taunt in the game, he can say the voice lines "Hello internet!", "Thank you internet!" and "Hugh Nation!", referencing this push. The showcase for Hugh's moveset, which was aired at EVO 2022, mentions Hugh being included as a "thank you" to the devoted Nickelodeon All-Star Brawl community.

Every single fighter intended for the base game of Nickelodeon All-Star Brawl, but not included in the reveal trailer, was leaked at some point:

• April O'Neil, Ren & Stimpy, CatDog, Aang, Korra were revealed on a silhouetted box art provided to retailers and revealed on announcement day. April's silhouette was vague enough that some people mistook her other characters, such as Carly from iCarly and the live action version of Dora the Explorer from the film Dora & the Lost City of Gold, but most fans deduced that is was April ahead of the reveal.
• Ren & Stimpy's inclusion was further proven by the discovery of an ESRB rating for the game, mentioning moves known as "Log Toss" (referencing a fictional toy from their show) and "First Fart" (referencing an episode of their show).
• A model lineup featuring Garfield was discovered from an unknown source before launch, it was confirmed to be real following the official reveal of Ren & Stimpy, whose models lined up with those featured in the leak.
• Toph was leaked by a rogue developer along with the stage line-up and scribbled out images of Garfield and Shredder. Garfield was not identified, but some players did manage to identify Shredder.
• Garfield and Shredder, who were intended for base game but had to be delayed as free DLC, were found inside the game's code through datamining immediately after launch.