Well, in the absence of any interesting answers yet, here's my thought.
The first few times I ever saw Jim Belushi performing, it surprised me how much he reminded me of Bill Murray, especially his looks. Now, if Murray is, as popularly thought to be, against any more GB films and as such an obstacle to any real progress of GB3, maybe Belushi could somehow fill in as a relative of Venkman or maybe even just a different character altogether. Without Murray's character to be the “mouthpiece” of the team, this would promote Dan Aykroyd's character to the position of “lead” or spokeman GB as it should be. Plus, it would be a nice bit of closure and tribute because Venkman was originally supposed to be played by John Belushi, Jim's brother.
Now I know there are bigger factors working against GB3 (Sony, et. al.,) but like I said, nobody's put up a better answer here yet!
- K -