Honestly I don't think they'd put one legacy character in a team of otherwise relevant characters, especially over names as big as the TMNT or Garfield.
It's not like Mr. Crocker or Plankton could particularly be considered "villains" in the sense the Nicktoons Unite games play them up like anyway, it wouldn't be too much of a stretch for the Shredder to team up with Binky the Clown from the prescedent of Vlad teaming up with a struggling restaurant owner, and later games dropped the Syndicate of Evil eniterly so I don't see why a new Unite couldn't have a single and/or original villain - hell, use a villain from another show not represented in the main team like Zim or Vlad.