I don't see a Spongebob cross-over with another cartoon. He lives underwater and most of the Nicktoons on Nick are above sea level. You might see Rocko, but the chances are small. However, they should do a Family Guy and Rugrats crossover. Stewie would take control of all the babies. I don't see any cartoon that would fit into a cross-over expect for the JN and FOP and maybe an Avatar and FOP cross-over would work too.