I'm absolutely of the mind that Cartoon Network, PTE's existence notwithstanding, is probably the biggest untapped well this game has.
While I do think we should get a few other shows first, I'm down for more Adventure Time characters, it's been such a huge hit, is relevant again, has moveset potential out the wazoo, and personally, I'm really fond of it. I think perfect rep would be Marceline and Ice King alongside Finn and Jake and that'd satisfy me.
Hell, same can be said for Steven Universe. I wouldn't flood the game with more characters from it, but no reason we can't either get Amethyst and Pearl to close out the team, or get some darkhorse picks instead like Lapis or Peridot.
One reason I want the Powerpuff Girls separated, beyond that just making more sense in the context of this game and who the characters are, is because I feel like PPG deserves multiple playable characters. If they're going to shove them together, the least they can do is also give us Mojo Jojo alongside them.
I think the characters I want most, besides the PPG, are Samurai Jack (I cannot for the life of me understand why he wasn't there to begin with), Dexter, Grim, Mordecai, Rigby, Courage, The Red Guy, any Ed, and Johnny Bravo. I also hope the leaks saying we're getting Craig are true.
Not a CN original, but if we get Space Ghost, it'd be hysterical if he were based off the Coast to Coast version.
ALSO, I feel it's worth bringing up Adult Swim.
I think Rick and Morty on their own are fine, so bring in ATHF characters and I am begging you, please put in some Smiling Friends characters! At least Charlie with that underhand gut punch.