Akroyd knows GB is his legacy,and unfortunately thanks to circumstances,(coughbillmurraycough) it never really reached it's full potential.I think (at least in Akroyd's mind) they should be on GB IX by now,handing over the reins a la other 80's classics (Star Wars,Star Trek,Indy Jones,etc) for an ongoing series.
I think it's another case of finding it hard to see the series exist without the original cast.I mean,Indy without Ford,Rocky without Stallone.In some cases it doesn't seem doable.GB without Murray (and now Ramis) just isn't that appealing.
For what it's worth,for a Ghostbuster TNG,this would be my cast:
Chris Pratt (the Venkman heir)
Zachery Levy (the Ray heir)
Kevin Hart (the Winston heir)
Emma Stone (the Egon heir)