PocoSlip

My thought process

Basically 100% Guarantees. The ones who are in both games so far

Spongebob and Patrick. They're literally just Mr. Nicktoon himself and his best friend

Garfield. Chris Pratt here is the closest thing we'll have to a guest character in the game since sure, Viacom owns the rights and stuff, Garfield hasn't had a Nickelodeon show yet unlike TMNT

Aang and Korra. The two main characters of arguably Nickelodeon's best show. Plus, the two play-styles contrast each other despite having basically the same tool kit, Aang is agile, nimble and light while Korra is hard-hitting and plays more aggressively

Lucy Loud. She got in over the main character for the sequel, I often compare it as if Smash Bros chose Isabelle over Villager. She's basically NASB's main Loud House rep as well as the main modern rep. Also I'm VERY biased.

Danny. I said it before and I'll say it again, Danny's the Link to SpongeBob's Mario. Ya can't have a Nickelodeon crossover without the two main Nicktoons Unite boys

Jenny. And to that extent, Jenny would be like Samus. Not as iconic as Danny but is still an essential addition in any Nick crossover, whether it's a fighting game, kart racer or a DnD campaign

Zim and Rocko. They kinda similar if ya think about it. A niche yet dedicated following, having movies years after their show's cancellation, they provide a play-style that not many characters could fill, and they both appeared in Smite.

Reptar and Ren & Stimpy. You could argue that they're this game's legacy pick similar to Mr. Game & Watch, Duck Hunt and R.O.B., considering they're the oldest original nicktoons in the roster (Remember, TMNT and Garfield were bought) while also fulfilling certain archetypes, Reptar as a heavy and R&S as stage control

Nigel. You are but a simple-minded fool if you think this absolute legend of a character should be cut from the roster. Plus, him and Reptar are the main Klasky Csupo reps.

Coming Back from NASB 1 (Ranked from most likely to return to least likely)

Toph and Sandy Cheeks. Two fan-favorites from their respective shows who were probably cut in 2 to avoid too much representation. But still, Sandy could be like Fox but hits harder and Toph fills in Earth that was missing with Aang's air, Korra's water and the two firebenders.

Leo and Mikey. Leo is the leader and Mikey is arguably the most iconic of the Ninja turtles

Lincoln Loud. He's the main character to Nickelodeon's most popular modern show. Though his best sister Lucy has more moveset potential, Lincoln honestly has the best alt potential. He could pull a Bowser Jr. and have different versions of himself as alts from Linka Loud to Warren to Ace Savvy (Assuming every character only gets 1 default and 3 alts like in 2)

Helga. Hey Arnold deserves a rep and Helga fits the most as a fighter.

Catdog. From what I've read, these two weren't able to return due to technical difficulties and a lack of time and budget to fix said difficulties. If that's resolved then they definitely would return, since they were the most requested cut NASB 1 veteran to return (As far as I'm aware)

Shredder. We needed more villain reps and Shredder is arguably the most iconic villain in the roster, other than Plankton. But if he stayed as a boss rather than a playable character, it wouldn't be the end of the world tbh.

Coming Back from NASB 2 (Ranked from most likely to return to least likely)

Squidward and Plankton. It's Squidward. And Plankton is the main villain.

Azula and Zuko. Azula is ATLA's villain rep while Zuko is a fan-favorite in series. They represent fire while Aang reps air, Korra with water and Toph with earth

Raph and Donnie. To finally complete the 4 turtles. But of they could only bring back 2, then I would personally go with Leo and Raph

El Tigre and Jimmy. Even MORE Nicktoons Unite reps

Ember. I feel like Danny Phantom deserves more than two reps. Vlad is the main villain while Ember is the fan-favorite side villain

Angry Beavers. They're Marvel vs. Capcom the character. For a show that hasn't been relevant for years, their concept and moveset are honestly so creative and fun... unless you're a competitive player

Gerald. Hey Arnold deserves at least two reps. Helga is already a great fit to rep the series and since Arnold is out of the question, his best friend is the second next best thing.

The Potential Newcomers (Ranked from most likely to least)

Vlad Plasmius. The most obvious newcomer. He's is the main villain on not only his home series but also NASB 2 specifically.

Casey Jones. If you think a non-turtle TMNT rep isn't gonna be a newcomer in a NASB gamem then you guys have the most creative mental gymnastics. April and Rocksteady proves that the turtles and Shredder aren't the only candidates for fighters. Plus, Casey Jones was in one of the concept art for NASB 1

Katara and Sokka. If Angry Beavers are Pyra and Mythra, then Katara and Sokka could be like Melee Zelda and Sheik where Katara is the more supportive zoner while Sokka is the aggressive rushdown swordfighter. ATLA is basically guaranteed to get a newcomer since it's tied with SBSP and TMNT as most represented series.

Timmy Turner. With his inclusion in Dice of Destiny, there's a chance the legal BS isn't as strong now as it was before, he could actually get included, considering he's one of Nickelodeon's most iconic shows. Plus, Hazel would be a GOATed alt

Bubble Bass. Not Bubble Bass specifically, but a new Spongebob rep in general. I chose Bubble Bass because he basically became part of the main cast in the modern seasons. He could play similarly to both WAAAARIO and K. Rool.

Sheen. He takes Hugh's role as the funny meme character from Jimmy Neutron while also representing the spin-off show. Yeah yeah, Planet Sheen is hated but it's still a part of Nickelodeon's library

Susie Carmichael. Mainly inspired by that one NASB 3 roster speculation video that was recently uploaded on YouTube. If Rugrats is ever gonna get new rep, Susie, might unironically have a shot considering her inclusion in both Nick Kart Racers and Dice of Destiny. She's using her All-Grown Up design to both avoid having players beat up toddlers and it also represents an underrated spin-off show.

Otis the Cow. DO IT YOU COWARDS!!!

Luan Loud. I said it before and I'll say it again, this game needs more modern reps, and Loud House deserves at least 3. I personally would have picked Lynn Jr. but one, she's the most divisive Loud sister and two, Luan has more moveset potential (Similar to how Lucy was chosen). She could play like Bugs Bunny and even Tom and Jerry from Multiversus, as this trickster trapper/stage control archetype

Queen Vexus. To compliment Vlad as the main villainess from her home series. Her and Vlad could honestly be like Bowser and Ganon from Subspace Emissary as the main playable villains of a story mode or something

Kitty Katswell. Representing the early 2010s and another Butch Hartman show rep. She could honestly have a play-style that's a combination of ZSS and Snake from Smash Bros

Scissors. The most out there pick in the newcomers section, I know. But hear me out! Firstly, Rock Paper Scissors is surprisingly doing good, having 3 seasons under its belt at the moment which is rare for a modern nicktoon these days.

The ones I don't see coming back unless they did a "Everyone is here!" gimmick like in Ultimate

Mr. Krabs. Out of these cut characters, he's the most likely to come cut. I just cut him to prioritize a Spongebob newcomer more as he's the most expendable out of the 6 veterans in my opinion. But if Spongebob wouldn't be getting newcomers, then I could see him coming back

Hugh Neutron. This will piss off the internet the most, but I actually think he could still work as just an NPC in a campaign mode, and Sheen would be replacing him as the playable Jimmy Neutron comic relief

Rocksteady. Even though I like Rocksteady, he could honestly just be an assist trophy alongside his boyfriend, Bebop.

Uncle Iroh. This one hurts me the most to cut since he's my favorite Nicktoon of all time. But objectively, it was weird that he was added before Katara and Sokka especially when 2 already had Azula AND Zuko as the firebender reps. But my GOAT can still cameo as a part of Zuko's ultimate like how Chrom was in Robin's Final Smash (We can use that one scene where Iroh demonstrates why he's called "The Dragon of the West")