As much as I love both Foster's and Spongebob I don't think that a crossover would do too well mainly because both cartoons are completely different in many different ways. For one thing, we've got one cartoon that takes place underwater while another one takes place on land. While I think that problem could easily be worked out, I just don't think crossovers of any kind are a good idea simply because they usually don't do too well when it comes to content and characterization-sometimes characters go out of character in crossovers (take the Lilo and Stitch crossovers for example. I'm sorry, but I didn't like that Proud Family/Lilo and Stitch crossover episode because some of the characters went out of character or were just flat-out boring).
Plus, one cartoon is owned by Nickelodeon while the other by Cartoon Network, so it will never happen, pretty much.
However, this is just my opinion.
