I remember reading an article from a year or so ago that not only do Aykroyd, Ramis, and Reitman control the rights, that Ivan Reitman has right of first refusal to direct and Sony wants "fresh eyes" on the franchise... which is another stumbling block.
I have also heard that no one is overly enthused about the script as it currently exists, which primarily stars Oscar Barrett-Venkman joining the business after Peter's death.