The reason I chose Ben 10(Teen version), Rex Salazar, and the Powerpuffs is because they are the pioneers of heroism of Cartoon Network just like Danny Phantom and The Avatar being the pioneers of Heroism in Nickelodeon. They are the symbols that can inspire even comedy characters like Ed Edd n Eddy and Spongebob to fight valiantly.
I would have made a plot as ambitious as Kingdom Hearts but that would make me write a big plot that I don't have time for. And even if I put Vilgax, Van Kleiss, V.V. Argost, Mandark, Mojo Jojo, Aku(to which I will anyway) then that would be too similar to Super Secret Crisis War a comicbook crossover with yeah totally CN characters.
Not my cup of ☕️ but they are very in right now so I wouldn't say it's out of the question for CN to do something like that as a big one-off special. Back about 10 years ago all their bumpers/ads were set in a Cartoon Network 'town' with all the characters interacting together so they've done similar things before on a smaller scale. OK-KO had some crossovers, too.