Make GB3 to HONOR Ramis
Seriously. This should be enough to get all of the guys back on board to make the film. Personally I think using the passing of "Egon" as a central plotline to bring them all together for one last adventure would be appropriate. It would honor what he did and bring closure to a fanbase hungry for more answers.
I don't see his death as meaning it should never happen, more that they should indulge in the fact that they have an opportunity to make a great film while simultaneously honoring an old friend.
Read this signature would you kindly?