First of all, someone said that GFIII was great because it drew on real events - all the GFs drew on real events. The whole story line from I to III uses real Mafiosa events to frame the fictional Corleone story.
Second, as far as story/film changes:
1. Get rid of Sophia Coppola, God bless her, her wine is good, but she cannot act.
2. Either explain the Pantangeli succession better or get rid of that story line and insert a Clemenza plot line.
3. Make Mike less "soft". The Mike from GFII would never have reconciled with Kaye - unbeleivable!
4. Show the Fabrizzio execution - we all felt better with that closure.
Maybe have GFIII show some flashbacks (as in GFII) of Vito's rise during the 1930s and then just before Connie's wedding. Perhaps have it mirror the plot in GFIII.