I decided to also do another roster prediction. I decided to do a 28 characters roster, which seems realistic but I wouldn't be shocked we would get less than that (I would be more surprised to have more than that). Here's the image and below my thought process on some characters:
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-Squidward: I'm pretty confident with his chance, he was always one of the most important Spongebob characters and I don't see why they would have put Sandy and not him.
-Mr. Krabs: I'm less confident with his chances than Squidward, but Spongebob is huge and I figured that if TMNT would have 5 characters Spongebob could probably too. To be honest if I had to delete one character and add a random slot instead I would probably delete him.
-The 4 turtles: like many point in this thread, there was never a game where only some of the turtles were playable, so I decided to add them all (by the way I just picked the first images on google images, I didn't bother finding the good turtles design)
-April O' Neil: I went back and forth between her and Shredder, but I thought that the roster might have not enough female character so I decided on her. I do think Shredder could happen, but I doubt we would get all of them and end up with 6 TMNT characters.
-Timmy: feels like an obvious choice, was always one of the most popular Nicktoons
-Ren & Stimpy: I add them as a duo, but I wouldn't be shocked if they're separate.
-Zuko: Avatar is hugely popular so I think we could see not only Aang but another character from this franchise.
-Dora: I'm guessing she's the one on the boxart? Honestly if it's not her then I don't think she will be in.
Characters that I've strongly considered but end up not putting them in the roster:
-Shredder (See April O' Neil section)
-Jimmy Neutron: I'm not sure why but Jimmy is rarely used in those crossover games, so I just dismissed him.
-Jenny Wakeman: I wasn't too sure on her chance honestly. While the show was popular, it seems to have less nostalgia around it than stuff like Ren & Stimpy or Danny Phantom, and unlike Winx club there's no revival or nothing new with this show so unfortunately I didn't put her here. If I have one more slot I would probably add her.
-Otto Rocket (from Rocket power): I feel like it could happen because of nostalgia (considering that we already have Ahh real monsters), but with such a small roster I didn't really know how to make space for it so I abandonned him.
-Really really big man (From Rocko's modern life): I feel like it could happen as a "joke" character like Powdered toast man, but again with such a small roster it's hard to find space for this type of character so I end up dismissing him.