The roster would cap out at forty characters on launch, with three thirds of them being returning veterans, while the remaining third would be brand new. An appropriate increase from the previous games' size while just teetering on the edge of modern fighting game feasibility. The veterans are evenly distributed across the games they appeared in, with nine veterans from NASB1 & 2 each and 12 coming from both games.
In tandem with the reveal of who will be in the first paid DLC pass, a poll would be announced where people online could vote for who they want to see get in the game next from a selection of fifty characters. The five with the most votes will be added FOR FREE as part of the second DLC pass, rounding up the final roster to a total of fifty characters, mirroring how Nickelodeon, as a network, is slated to celebrate its 50th anniversary in 2029.
Many of the inclusions you see above are done with the goal of orientating the roster more towards representing the wider history of Nick's ORIGINAL properties and characters, helping the game to feel like a celebration of all eras and a pedestal to spotlight some of their lesser known properties, like I believe it should have been from the start. This train of thinking is why there's more new series featured, the TMNT cast are redesigned to fit their 2012 and Rise incarnations made in-house by Nick, and Garfield is saved as potential free DLC.
Shown in the three tiers below the yellow one are characters who would serve various non-playable roles. Leni for instance would be a shopkeeper selling new alternate costumes for in-game tokens and/or slime, Henry and June would be the announcers, Rudy Tabootie would help Snap by drawing platforms and props to fight opponents off-screen through use of his magic chalk (akin to the person using the zapper in Duck Hunt's moveset from Smash), and, for the ultimate villain seeking vengeance against the Nickelodeon All-Stars, I can't think of a better candidate than Vendetta, the fiend-making little girl whose show was arguably the most mistreated by the network.
The final tier below that shows characters such as Abrasive SpongeBob and Hazel Wells from Fairly OddParents: A New Wish, serving as alts for fighters, most of which have unique voice lines and slight animation changes to reflect their personalities, akin to how the Koopalings were alts of Bowser Jr. (which is the direct inspiration I have in mind for how the Rugrats would play) or the variant skins from Multiversus.
All of this might be wishful thinking or outside the realms of possibility, given the small budget Nick gives to developers for these newer crossover games. But if this were how a NASB3 were to play out, would any of you be satisfied and/or interested? If you have any questions, please let me know.