4. The Godfather Part IV
The Movie: After The Godfather Part III earned seven Oscar nominations, director Francis Ford Coppola and his co-writer Mario Puzo (who also wrote the original Godfather novel) began planning a fourth film.
The Godfather Part IV would feature more flashbacks to Vito (Robert De Niro) now in the 1930s, the return of Vincent (Andy Garcia), another glimpse of Michael (Al Pacino) before his death, and Leonardo DiCaprio playing a young Sonny.
Why It Got Cancelled: Puzo died in 1999 before the script could be written, and the film died with him.
Given that Part III is universally accepted to be a far weaker film than its predecessors, it didn't seem particularly likely that Part IV would return to that former glory, especially if Al Pacino's role was limited to just a few scenes. This was definitely the right call.