Having Tom Hagen die of cancer in GF III would have been an interesting parallel to the death of Genco Abbandando in GF I (the scene of which of course, didn't make the final cut.)

Maybe early on in the movie, Michael and Tom would be talking over the olden days, and flashback to the Genco scene and set up Hagen's own deathbed scene later in the movie.

Well, who knows.