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's files have a set of unused placeholder models, all in a significantly more realistic style than the game and not based on any Nickelodeon IPs, comprised of a female pirate model from Adobe's Mixamo service; pirate, military, and ninja-themed props and weapons with unknown sources; and a "dangerous pumpkin" taken from a Unity asset store pack, which also has a leftover UI graphic alongside multiple other asset store props.

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.

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.

In a sponsorship segment for Nickelodeon All-Star Brawl produced by comedy group Dude Perfect, a flat, horizontal variant of the game's logo is used on a banner. This is the only known use of this logo during the game's promotion.

Two of the first direct-feed pieces of gameplay released for Nickelodeon All-Star Brawl were a 1v1 between Nigel Thornberry and April O'Niel, and a 1v1 between CatDog and Lucy Loud. Both of these battles were wildly one-sided in favor of April and CatDog respectively. In the battle against April, Nigel was controlled by a playtester who had never actually used Nigel before; while in the battle against Lucy, CatDog was controlled by Mattias Hakulinen, who - while not a competitive player - was the best NASB player on its development team at the time. It is not known who was playing as April and Lucy.

One divisive element of Nickelodeon All-Star Brawl is how hard it leans into competitive play, which has been perceived as coming at the detriment of content aimed at casual players such as items and a story mode. According to Ludosity CEO Joel Nyström, Nickelodeon specifically wanted the game to be able to attract a competitive audience, and chose Ludosity as a lead development team due to the positive reception of their first platform fighter, Slap City, from competitive Super Smash Bros. players. 2 years later in 2023, the game's gameplay designer and community manager Thaddeus Crews would respond to a tweet from YouTuber TierZoo complaining about a focus on complicated competitive techniques in non-Smash platform fighters, as well as some replies to the tweet complaining about 1v1-based platform fighter design as a whole, with a clarification on why Ludosity's platform fighters have been marketed so directly at competitive players, as well as why he feels other games in the subgenre are marketed as such:

Ren & Stimpy were made a duo fighter in Nickelodeon All-Star Brawl to allow them to interact with one another, something that wouldn't be possible if they were two separate characters. Ludosity CEO Joel Nyström noted that it also allowed development time to be assigned to one extra fighter.

In December 2023, information spread about Warner Bros. Discovery, which owns Cartoon Network among other major animation brands, potentially merging with Paramount Global, which owns CN's long-standing TV animation rival, Nickelodeon among other animation IPs. Although this deal would not come to fruition at the time, Nickelodeon All-Star Brawl game designer Thaddeus Crews commented that he would take the chance to work on a Cartoon Network vs. Nickelodeon fighting game "in a goddamn heartbeat".

Out-of-bounds on Omashu are two floating textureless rectangles, while Space Madness has an out-of-bounds cube. These were both possibly used during development.

Hugh has an unused model of a spear, referencing the popular caribou hunting chant scene from The Adventures of Jimmy Neutron: Boy Genius episode "Jimmy on Ice". While it is fully textured and loaded into the game whenever Hugh is, it is not used.

Various models and textures of Ittle Dew from Slap City are inside the files of Nickelodeon All-Star Brawl, as she was used as a placeholder during development.

In launch versions of Nickelodeon All-Star Brawl, Garfield's stage Sweet Dreams was discovered before his official announcement in an unfinished form which takes place at day time, unlike the final version which takes place at night. Additionally, a thumbnail was discovered for an extremely early version of the stage that takes place at sunset and has an opaque lasagna tray. When Sweet Dreams returned to Nickelodeon All-Star Brawl 2, under the name Food Dream, the daytime setting for the stage would be restored.

In the final update of Nickelodeon All-Star Brawl, Patrick's command grabs gained a glitch that made them unable to penetrate through shields, negating their primary purpose. When asked about fixing Patrick's grabs after the launch of the game's sequel, gameplay and character designer Thaddeus Crews responded "I would if I could".

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.

Lincoln Loud's neutral water gun move "The Splasher" had to be reworked during development; the original version of the move stunned opponents and could be cancelled into most actions in the game. This was a flawed move design however, as players could simply cancel the move into a waveland and then perform it again, making an infinite combo. If this move stayed in, Lincoln would have had to be banned from early competitive play, much as Michelangelo would end up being. In the final game, the water gun pushes opponents and cannot be cancelled.

During development of Nickelodeon All-Star Brawl, Patrick's gravity accidentally ended up being set to 420 in certain unspecified circumstances. Thaddeus Crews stated that it was kept that way as a joke.

Nickelodeon All-Star Brawl's movesets are modified through mathematical functions. Due to the monotony of assigning these values, certain modifiers' values are set to be pi solely as a joke. Thaddeus Crews, who added the instances of pi, stated in a Reddit post about the use of the number:

When used on the Wild Waterfall stage, the Suck-O-Matic can suck up the log platforms, altering the layout of the stage.