5th Ghostbuster: Technically if they want it bad enough… sure. But it seems that Aykroyd was the only one really pushing for it, Ramis was just along for the ride. However, don't forget about Joe Medjuck, Bernie Brillstein, and good ol' Bill Murray who own (equally) one-fifth of the Ghostbusters pie. I think Ivan Reitman cashed in on legal rights to the series as well- I know he did with the two animated series' but as for the films I'm still uncertain.
About 98.98% sure that the rights are still split 5 ways though.