My ideas:
-cut out that ridiculous ragtime music when Hagen visits Hollywood!! (this is on a par with the annoying March of Time music in Citizen Kane...)
-Cut the wife-beating scene in the first film to half its length
-Improve and shorten the beginning of the modern section of Godfather II. And replace Rocco and Al Neri with competent actors!
-Make Hyman Roth live in a house that doesn't look like dull and depressing '50s suburbia
-Make Michael wear a dark suit all the time in GFII!!!
-Give FFC 6 more months to make GF3, so it wouldn't have turned out like a rough draft because of commercial pressures!