The Godfather Part IV

The Godfather Part IV was a planned sequel film in the Godfather series. It was cancelled when director Francis Ford Coppola's cowriter Mario Puzo died in 1999.

Plot
Unknown.

Why It Was Cancelled

 * 1) Mario Puzo died in July 1999 before the film could be launched into production (mentioned earlier).
 * 2) Francis Ford Coppola was very reluctant about returning to the Godfather franchise despite critical acclaim.

Result

 * The film was permanently cancelled.