Jimmy Neutron world would be CG so it would feel like an alternate universe. It would would be too off canon along as well as being too expansive. Fairly Oddparents had one of Danny Phantom's character cameo so it is possible. May have to explain the mishaps
Well, the crossover they had showed they were separate dimensions, so it would make sense for only one of them to be of the same world. I'm gonna go with Fairly Oddparents because it's 2-D, while Jimmy Neutron was 3-D, something which if I remember right was noticed by the characters in a couple jokes, everything either being all bulgy or flat depending on which world they were in.