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.

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 lead 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.

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:

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.

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 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.

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.

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.

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.

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.