The first rule of showbiz is "leave them wanting more." This rule was nearly broken by GF III and another installment would kill it. In most cases one sequel is one to many. Look at Legally Blonde, Meet the Fockers, the endless Terminator movies, Star Trek , Star Wars, and many others. Who on earth wants to go see an aging Andy Garcia trying to run the family in the '60's and 70's with the help of a broken Michael?

"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"

"I stood in the courtroom like a fool."

"I am Constanza: Lord of the idiots."