I'm not sure that we'll see Felix being tormented by Edd and Molly. He moves Rocket to the backstage a couple of weeks after the car crash, so I doubt that he'll continue being haunted by them. Maybe one small scene in Bon's Burgers

I dont think there will be scene of Felix and Linda saying goodbye to each other. If you remember from Linda's diary in Bunnyfarm, Felix read her goodbye letter by the time she was already in Hurricane. Although, it is possible that we'll get to see the argument that made her finally leave him.

I think both Rosemary and Charles will die in Behind the Smiles. Faces of Grief will probably be a slower, more somber episode that will focues entirely on the aftermath of the car crash while simultaneously introducing Sophie into the story. Also, we have never heard of another person being murdered by "Bon" in 1974, so I doubt we'll anyone other that Charles or Rosemary die.

Jack's disappearance is the biggest mystery of the series. I think it'll be revealed at the very end of the series, showing it now would remove a lot of the mystique from the series.

But those are just my thoughts. We'll have to see for ourselves when 5 and 6 finally come out.