Nickelodeon All-Star Brawl 2 has been noted as having almost zero similarities to the game it is a sequel to from a gameplay perspective. Gameplay and moveset designer and community manager Thaddeus Crews would address this disparity in a short Twitter thread shortly after the sequel's launch by reassuring fans who prefer the first NASB that the sequel was not intended as a replacement for the first game:

Nickelodeon All-Star Brawl has an unconventional design mentality that contributed to the game's divisive reception. This was the result of the development team having very little time and resources to brainstorm due to tight deadlines, and the core gameplay implementer on the team early in development not wanting to retread their prior work on Slap City (a more conventional platform fighting game closer to the design mentality of Super Smash Bros. Melee). Community manager and game designer Thaddeus Crews noted that the absence of moves such as forward tilts, side specials, and back airs restricted what could be done with designing movesets. While he was initially upset by this, he came to accept the game's nature as it developed a small loyal fanbase, some of whom continued making unofficial patches for the game that Crews endorsed as the team was no longer allowed to continue updating the game in early 2023.

In the release version of Nickelodeon All-Star Brawl, the stage Royal Woods Cemetery from The Loud House contained a hazard where tombstones would pop out of the ground, sending any character beneath them flying and remaining as a wall for a short while afterwards. In version 1.5.0, the update that added Rocko as a playable character, this hazard was removed from the game completely, making the stage into a flat floor, similar to Final Destination from the Super Smash Bros. series. The reasoning for this change is because the stage that was intended to serve as a Final Destination parallel, Harmonic Convergence from The Legend of Korra, was considered too long for use in competitive play, and was removed from this distinction in the same update.

When the email from Nickelodeon proposing a Nicktoons platform fighter was first sent to Ludosity, the company's CEO Joel Nyström initially dismissed it as spam mail.

Originally, Reptar was going to spit cereal from his mouth, but Nickelodeon insisted that he shoot fire instead.

If the player holds the button after performing Hugh Neutron's grounded up light and then presses left or right on the stick, they can skid Hugh very slightly. Hugh moves slightly further with every skid, and, as there is no cap to how far the skids can go, he can become so fast that he teleports from one side of the stage to the other with a single skid. This has no practical use in a battle due to the amount of skids required to begin teleporting, as well as the skid count restarting every time Hugh uses the move, but appears to be a deliberate feature implemented by the developers, as every time Hugh skids, he makes a short squeaking noise.

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.

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.

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 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.

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.