I actually had kind of a weird dream last night. I blame AMC for its repeated airings of the first two movies.
I know that Andrew Hagen became a priest, but he had another son (Frank?) and two daughters, right?
This would probably be a very boring movie, but I had the idea that maybe Hagen's body would finally be discovered and identified maybe by dental records in the late eighties/early nineties, which could lead a couple of Tom's grandchildren to launch a private investigation. Maybe Tom's kids had been told that he had died in an accident or something like that, or that's what Frank and the sisters told their kids in an attempt to leave the "Corleone" era of their lives dead and buried.
Anyone think this isn't a totally worthless idea for a little bit of fun writing?
My favorites were Vito and Tom, but I do have a soft spot for Michael, too.
Here's another potential plot twist. It could turn out that Michael and Tom share some grandchildren. I can see Anthony getting somewhat bitter after Mary's death and Michael's withdrawal from the world, changing his name to "Tony Adams," and maybe wind up marrying one of Hagen's daughters eventually. One of the secondary draws could be the fact that he doesn't have to talk about his family with one of the Hagen daughters.
And just to continue the ol' Godfather tradition, maybe it opens with a welcome back celebration for Father Andrew returning to the States after years of service in Rome.