Garfield and Shredder have actual files in the game that indicate they are going to be made playable - consider them confirmed. Rocksteady has files indicating that he was briefly being worked on before being scrapped, assumedly for Shredder or April. There is also a Double Dare stage that is being worked on, probably without a character attached but who really knows.
There are also announcer clips for other characters, which includes: Angelica Pickles, Avatar Roku, Azula, Barnacle Boy, Beebop, Casey Jones, Chuckie Finster, Cindy Vortex, Dark Danny, Donatello, Donnie Thornberry, Eliza Thornberry, Filburt, Foot Clan, Gir, Jet, Jimmy Neutron, Katara, King Bumi, Krang, Mermaid Man, Metal Head, Mr Krabs, Plankton, Prince Zuko, Raphael, Ren Höek, Rocko, Rock Steady, Shredder, Skulker, Sokka, Splinter, Squidward, Stimpy, Susie Carmichal, Tommy Pickles, Uncle Iroh, Teenage Mutant Ninja Turtles, and Vlad Plasmius.
These voicelines are just placeholders and nobody from them should be taken as confirmed - it sounds like Ludosity wasn't even involved in their recording and Ludosity are the ones pitching characters - but the fact that wheoever did choose to record them thought Rocko's Modern Life and Jimmy Neutron were IPs eligible for inclusion in the game is a good sign for them at least not having too much red tape.