I am guessing Tak would also be an issue given his shared ownership with THQ. I know he isn’t the most requested, but part of me would like to see him as he was an interesting experiment in Nick history, had good games, and I just view him as part of the main 6 thanks to his large involvement in Attack of The Toybots and Globs of Doom.
Kappa Mikey is my personal most wanted which makes it sad to hear his fate, along with FOP,a big part of my childhood.
Other than that, the only other rights debacles would be film characters as Paramount is a bitch with rights. SpongeBob could never utilize the film characters in the show for the longest time due to Paramount. I think Otis is fine seeing as he was going to be in a canceled kart racing game, but characters like Rango might be screwed even though Nick made the movies.