I've been thinking about this for a while, and I think that these are crossovers that have YET to be tackled. Plus they could probably offer some lucky newbie his first fanfic plot:

SPIDER-MAN: The Ghostbusters and Spiderman team up to stop both an ectoplasmic being and one of Spidey's villains. Maybe Master Spider would be interested in this.

DRAGON BALL Z: If Ectodude can make posts about a Sailor Moon-GB crossover, then DBZ has options too, if you've seen the series.

MEN IN BLACK: A secret AlienBusting agency and the Ghostbusters teaming up. What are the Odds of the outcome?

PHANTOM INVESTIGATORS: Everyone here at the site hates this series SOOOO much. Why take out the series in a story where they can't handle a major ghost (not one of those pansywaists they have fought so far) and have to call in the pros: the Real Ghostbusters.

X-MEN/X-MEN EVOLUTION: Either would work.

BATMAN: Another hero of NYC.

SEPT 11.: I think it would be a good thing to see the horror of 9/11 through the eyes of a GB.

This list is only half done. If anybody is willing to do this, contact me and I will gladly give aid.

lol! How ironic! I was thinking of doing a 9/11 RGB story where the planes were ghosts and the RGB's busted the planes and saved everyone or have the RGB bust Osama Bin Laden! lol! But I was afraid that it might offened people, re-open their wounds you know.

Part one of my 9/11 crossover is already in the fan fiction section.
(Black Day at the Firehouse).

I am in the process of writing a Spider-Man/Ghosbuters crossover.

Basically any Cartoon on Cartoon Network:

Courage the Cowardly Dog: Have Courage call in the RGB's

PowerPuff Girls: Have a ghostly Villain the Girls ain't able to handle

2 Stupid Dogs: Well that basically explains my Twister series..

Looney Tunes: Wide Option here, try to bust Marvin the Martian or something..

Johnny Bravo: Now that would be a show, him and Peter could fight over girls or something

Dexter's Labratory: A spirit rises in the Labratory…

Scooby-Doo: Let the RGB's show the gang how its really done!! The Ecto-1 could beat the Mystery Machine anyday!

There's many more but my train of thought has just derailed itself..so, yeah..there ya go

I just had another flash of brilliance: You know that scene in CASPER that had Dan Aykroyd running out of the house saying ‘Who Ya Gonna Call? Someone else!’? How about a reversal? The guys actually busting the Ghostly Trio!
Nuff said.

You know.. I only liked the movie of Casper because of that one scene…. That's the truth.