In 2023, a set of extremely early Nickelodeon All-Star Brawl screenshots were shared through Discord by a developer, showing an aesthetic closer to Slap City and early versions of SpongeBob, Oblina, Leonardo, The Dump, Jellyfish Fields, and - most notably - Rooftop Rumble, based on Shredder's lair from the 2012 Teenage Mutant Ninja Turtles show instead of the 80s TMNT (despite the 80s turtles having already been decided on this point). Also among the images was Hardcore Chores, Rocko's DLC stage, suggesting that a Rocko's Modern Life character may have been intended for base game.

A Twitter post made on the Nickelodeon All-Star Brawl Twitter upon the release of the Garfield DLC shows that his bib alternate costume was originally supposed to be plain white with stains of lasagna tomato sauce instead of clean with red checkers.

Christopher Near, the artist who painted the box art for Nickelodeon All-Star Brawl, posted an image to his Instagram of concept art for the game which predates the game's development and was intended to excite Nickelodeon executives over the concept of a Nicktoons fighting game as opposed to influence the product itself - this poster featured a few characters who would not appear in the final game:

• Jenny Wakeman and Rocko, who would appear in the DLC.
• Jimmy Neutron and Raphael, who would appear in the sequel.
• Casey Jones, Krumm, and Angelica Pickles, who have not appeared in a NASB game.
• Wanda from The Fairly OddParents and Reggie from Rocket Power, two shows that have never been in a NASB game.
• Stimpy as a solo character instead of a team with Ren.

Interestingly, despite predating the game's development, it features 80s designs for Rapheal and Casey Jones despite the eventual developers of the game saying that Nickelodeon wanted the 2010s Turtle designs.

Jellyfish Fields, Harmonic Convergence, and Duck, Duck, Pie! all have unused early level geometry. These are all loaded in when the final stages are played and are simply hidden from view. Harmonic Convergence is very close to its final form; Jellyfish Fields lacks textures, but it can be seen that it has the sign on the base platform instead of the background; Duck, Duck, Pie! was originally a completely different style of pie that has its filling visible, which was likely changed to prevent depth perception issues.

Powdered Toast Man has an unused animation where he farts and blushes. This animation may have been intended to be a taunt performed while strafing.

3 items from Nickelodeon All-Star Brawl were removed before the items update: a fart cloud, the Krusty Krab blimp, and a wrench set. The fart cloud and blimp's use are not fully known, but the wrench set would've created a platform like Toph's down special. They were rejected "because they weren't that fun".

In the files of the game, there is content implying two other potential appearances for the Krusty Krab Blimp: a texture for the Kids' Choice Awards blimp, and voice lines from the announcer referencing the Teenage Mutant Ninja Turtles' blimp, which is used in April's up special, as an item.

Nickelodeon All-Star Brawl has an unused file of the logo of Nickelodeon's parent company, ViacomCBS (now known as Paramount Global). In the final game, ViacomCBS is only mentioned by name in the credits instead of with a logo, and the opening of the game shows a Nickelodeon logo instead of a ViacomCBS logo.

On the base platform of Irken Armada Invasion, there is a monitor that displays a static image of the Irken insignia. There are files suggesting that this was originally going to be an animation rather than an image, as animation textures exist for a set of clips of the Almighty Tallest taken directly from Invader Zim episodes, and there is an unused how-to-play graphic showing one of said clips on the monitor. It is not known if these clips were planned to be used or if they were placeholders for another animated graphic, and whether they were scrapped for licensing issues with TV footage, distraction issues on a stage made for competitive play, or something else entirely.

In the files of Nickelodeon All-Star Brawl, one can find concept art graphics for NPCs and objects on the Sweet Dreams stage, including the walking food that would appear as enemies and in Garfield's super move in the sequel. The cotton candy sheep is pink in the concept art, but white like a real life sheep in the game. It is not known who illustrated these graphics, and whether they were from Paws Inc. or the NASB development team.

According to voice director Douglas Carrigan, of the characters whose voices had to be recast for Nickelodeon All-Star Brawl, Oblina was one of the hardest to find a replacement voice for after the death of her voice actress Christina Cavanaugh in 2014:

In the pre-day 1 patch version of Nickelodeon All-Star Brawl, Toph had a different taunt, where she raises her arms in the air. After the patch, this was changed to her waving her hand in front of one of her eyes.

Although Nickelodeon interfered signficiantly with the development of Nickelodeon All-Star Brawl, moveset and gameplay designer Thaddeus Crews has stated that he was surprised and grateful for the amount of creative control the game's teams were given:

Aang and Korra were first revealed for Nickelodeon All-Star Brawl on the Xbox Podcast with footage from their respective trailers. The CGI portion of Korra's trailer was not played, and following the reveal, Aang's trailer would be posted to YouTube, but not Korra's. This led some to believe that a CGI trailer for Korra was never actually made until May 2022, when a YouTuber named socks dude discovered the trailer on the website for i95Creative, the studio that edited the promotional material for Nickelodeon All-Star Brawl.

When it was announced that Nickelodeon All-Star Brawl's paid DLC characters were due to be revealed, it was with a tweet reading "Back to the lab again". Some theorized this was a tease for either Jimmy Neutron or Jenny Wakeman, both highly-requested characters with a technology theme. However, this seems to actually be a reference to a livestream by Super Smash Bros. Ultimate YouTuber GimR from two days earlier. GimR advertised the livestream for days prior, promising a new technique that would change the game's competitive scene, only to fail to actually start the stream due to technical issues. During the time he was live, he played a looping intermission animation depicting him as Dexter from the Cartoon Network series Dexter's Laboratory and playing the song "Back to the Lab" by Prince Paul and Wordsworth for over an hour. This mistake would become a meme within the Super Smash Bros. community for a short while afterwards. Ultimately, Jenny, Rocko and Hugh Neutron, the latter two not being related to technology, were revealed, and GimR would separately reveal the promised new technique, the "Slingshot", two days after the failed livestream.

In Jenny's showcase, her neutral air light attack is referred to as a "lingair". This is a reference to a tweet made by developer Thaddeus Crews prior to her reveal, where he questioned the appropriateness of the "sex kick", a competitive fighting game term originating from the Super Smash Bros. series for an aerial attack that stays out for an elongated period and slowly becomes less powerful, and suggested "lingering air" as a sanitized alternative.

Michelangelo has an unused animation where he gets down on his knees while pumping his fists in celebration. The animation is unfinished, as one of Michelangelo's fingers unnaturally twists into his palm. The animation is featured in one of the CGI segments in the game's reveal trailer, where he slides forward while doing it. This could have been an early version of his grounded down light move, Air Guitar, which is similar thematically and propels him forwards in the same way.

Previews for new fighters being added to the game showed that one of Jenny's intended pre-battle quotes was "You wouldn't like my brain! It's all circuity and metallic!" While this line was taken from an episode of My Life as a Teenage Robot, it was removed from the final game due to it being associated with a pornographic parody of that episode by Internet animator Zone, which edited the line to replace "brain" with "body."

Thaddeus Crews, a member of the development team and community manager, initially expressed disgust in a now-deleted tweet (the tweet he replied to also having been deleted) that the line was in the game and announced its immediate removal. He later explained that the line was likely added due to a lack of awareness about the parody, the end result of the team being "spread out and not on the same page", while adding that "Most likely someone on the team did know what they were doing by adding it", despite Crews himself being previously aware of and publicly associating the original line with the porn parody prior to its removal from the game.

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.

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.