Outside of the ones you listed, I imagine another SpongeBob character would be on the cards for DLC. It's Nickelodeon's flagship series, and there are plenty of viable options from Squidward to Plankton to Mr. Krabs to even someone weird like Mermaid Man. Personally, I see it being Squidward, since he's the third most important character after SpongeBob and Patrick (plus all the day 1 memes about him, which tells me that people really wanna see him in the game), with a moveset revolving around long limbs and a few musical references in his specials.
Other than that, I can definitely see them moving on to some lesser known Nicktoons after the big names are cleared away. Snap from ChalkZone I can oddly see happening, and assuming Rocko gets in, Angry Beavers would be the only noteworthy 90's Nicktoon left. (I'm not counting Rocket Power since its first episode aired in August of '99. If Rocket Power's a 90's show, then I'm a 90's kid.) A third Loud House rep MIGHT happen since the show is so popular these days, but I also don't know how much the devs really care about The Loud House and Nick doesn't seem to be pushing any "please shill this" picks onto them. Shredder would DEFINITELY be a DLC pick, so I think we're pretty likely to see him at some point, even if we don't get the rest of the turtles.