The Grace Hamilton character was intended as kind of a parallel to Kay Adams - a non-Italian, WASP-establishment character for the Don-apparent's love interest.

Then, of course, there are direct parallels between Mary Corleone and her late step-mother Apollonia, even down to the circumstances of her death.

As Michael returned from Italy and the murder of his young love Apollonia to marry Kay, certainly in any GF IV plot, after the murder of Mary, Vincent would probably pick back up with Grace Hamilton. A mafia Don being married to a scoop-hungry reporter -- that would certainly set up some conflict points...