Leaping Lumbago!
Chance: 5%
I don't see Rancid Rabbit happening over, say, one of The Greasers, who are portrayed as fighters and bullies as opposed to the less physical and more methodical Rancid. Yeah he's 90's and Nick has a fairly limited pool of potential additions, but like, does CatDog really need another character? No one's asking for another CatDog character, and if they were, I don't think Rancid Rabbit would be on the top of the pecking order.
CatDog themselves works just fine as a unique inclusion and that's enough.
Want: 5%
Full disclosure, I also hate CatDog. Say what you want about Invader ZIM, but at least it was visually and audibly stylistic, had characters who deserved their misfortunes while still being entertaining, and above all, it was actually funny. CatDog was none of those things. It was an absolutely miserable show. Everyone was a jerk to CatDog, Cat and Dog were jerks to each other, almost every episode was either "bad stuff happens to CatDog and then it ends with something even worse happening" or "good things happen to CatDog for once only for it to be cruelly and brutally yanked from them at the end". A good load of the time there isn't even any humor in it, it's just overwhelming misery porn.
Also if I'm honest, alongside not being likeable at all, none of the supporting characters from CatDog are even all that memorable. CatDog themselves are remembered through their design and concept alone, not for anything that actually happened in their show. If people weren't maddeningly curious about how they poop, would anyone even remember this character at all?
CatDog by themselves is enough.
Chef Gordon Bleugh prediction: 2%. Still more likely than Goku.
Noms
5 for Katara
5 for Purple and Brown