It's actually Dan Ackroyd, Harold Ramis and Bill Murray who have been pushing the new younger recruits in all the interviews I have seen. They are all pushing towards 60 now and only Ernie Hudson looks young. There is no way that in real life or make believe that you would expect them still to be busting Ghosts at there age, just look at how bad Indie 4 was.
I would of expected them to of moved on a bit, but not too far like they did in GB2, mind you they were forced to in GB2 as they were shut down. I would of thought that after a few years they would of made enough money to hire people to do there work for them.
They then should of moved on in there own lives like get married had kids, woked in other areas like research & Development and running there Franchises. Then you can pick up in GB3 with some story which would feature the original GB's having to help out a new supernatural problem.
They also need to move away from the usual formulae that they tried to copy in GB2 from GB1 (the whole Stay Puft & Statue Of Liberty endings).
You don't want to sit there and see similar plot points and events from previous films. The thing I liked from the first film was that they gradually ramped up the tension and action from each scene to scene, just when you thought that must be it they can't top that, then something else happens.
The story and characters need to be good, you should not try and include everyone from the previous 2 films if they have nothing to add to the story to progress it. When you think that the film will be like 2 hours long you have to have enough dialogue and story for the characters in the new film, simply swamping it with too many actors will not give enough time for each person to have enough screen time.
I would be happy if the new recruits were mid 30's and older, I to think that very young so called stars like Shia just can't cut it.
Damn! just noticed I started to go into a bit of a ramble there, lol.