-Scooby-Doo meets the Ghostbusters:
Both mediums deal with ghosts (one solves mysteries mostly involving ghosts and the other captures ghosts). The story could have the Scooby gang in New York as they encounter a ghost that turns out to be an actual ghost and run into the original ghostbusters and they work together to solve the mystery. I mean, hey, if they did a crossover with Supernatural it's not so far-fetched.
-Ren and Stimpy meets Snookums and Meat:
As I have learned, Snookums and Meat was basically a Disney clone of Nickelodeon's Ren and Stimpy (I've seen various clips of the show and the evidence shows). The way this crossover could happen is the original duo encountering the Disney clone and the plot would be sort of similar to how they did the crossover between Family Guy and the Simpsons.