Godfather 4 would be simple. Andy Garcia as Vinny in the modern day, watching the Corleone empire go down the pan with his hot-headedness and his fathers taste for the narcotics business, and Robert De Niro as between GF1 and GF2 Vito charting the rise of the family and the wars of the 30's with the Irish. Throw in a few stories about Luca Brasi, and hey presto a fantastic film. You could pad it out with Di Caprio and Ribisi if necessary.