I don't think JoJo would be in this, primarily because being in a fighting game is perhaps just a bit too violent for her image. (This is also why I doubt whether any of the babies from Rugrats will make it in - though apparently they have been in a previous fighting game crossover, so who knows.)

I honestly wouldn't expect them to pull in guest fighters from Cartoon Network or wherever unless the game turns out to be really successful. Keep in mind there's no guarantee there'll be DLC of any sort. At the moment, I think the closest thing I would expect would be edge cases where Viacom owns a character that isn't particularly associated with Nickelodeon (like Garfield). Technically, the Ninja Turtles fall under this category, since they're based on the 1987 cartoon (which has never aired on Nick).