Personally, here are the sort of likelihood tiers I see for potential newcomers.
S Tier Likelihood: Squidward
This one is fairly self explanatory. The entire reason we're talking about a potential sequel is because of the supposed leak, and if you think it's real, then that would mean Squidward is in the game.
A Tier Likelihood: Jimmy Neutron, Zuko
These are the two other newcomers that, in my opinion, have the most going for them aside from Squidward. They're some of the most requested characters that didn't make it into the first game, and asset reuse from Kart Racers 3 is possible should the need arise.
B Tier Likelihood: Donatello, Raphael, Odie
When it comes to further expanding existing series, I think these three stick out. Donnie and Raph aren't as requested as the three above, but they're super close, and their omission is very similar to Jimmy or Squidward in that it really stuck out. Odie, meanwhile, has the necessary pedigree (hah), but Garfield being a DLC addition last time didn't really leave a spot for Odie. That can change now.
C Tier Likelihood: Mr. Krabs, a Rocket Power character, Vlad Plasmius (or any other Danny Phantom antagonist)
Mr. Krabs has an okay shot I think. I just put him lower because we already know we're getting Squidward should the leak be real, and if so, I'm not sure they'd double dip here considering how conservative the first game's SpongeBob roster was. With Rocket Power, I think it's the most likely new series to be represented, but that's not saying much. I think most additions would be from existing series. As for who, they could go for Reggie for a more traditional rep and a new female character, or they could go for Tito for a fan favorite silly pick. Vlad is a reasonably popular request, and is a good choice if they want a villain (as Plankton would reasonably be behind Squidward and Krabs in the pecking order).
D Tier Likelihood: Arnold, Clyde, Dib or Professor Membrane, Plankton (+ Karen), Katara, Sokka, Rudy and/or Snap, Dagget and/or Norbert, Henry and/or June, Otis
This is now getting into "will only really get in if we get like 10+ newcomers" territory. These are the longshots. The wildcards. This is where I put all non-Rocket Power new series additions, more low-key additions to existing series, or in the cases of SpongeBob and Avatar, characters that aren't Squidward, Zuko, or Mr. Krabs.
F Tier Likelihood: Live Action characters, Timmy Turner (or other Fairly Odd Parents characters)
The former were explicitly disconfirmed during the first game, and Dan Schneider's legacy's continued dive into the abyss only further strengthens the idea of them never being acknowledged here. And for FOP...well, the writing is on the wall.