I've long thought a sequel to the Godfather trilogy was obsolete, nonetheless recently I've started to think some otherwise,

because to me, you absolutely need Al Pacino in a sequel, and as we saw him die at the end of the 3rd, that rules out any possible return, but we saw him die much older then he was when Vincent Mancini took over,

having read "the omerta" (which had some good ideas, but was but so/so in the end), some of it's ideas could be recycled as sequel of the Godfather,

we could have Michael Corleone form a "secret" champion for the Corleone's, a sicilian from their hometown (who as youngster whose father has been murdered, to echo the start of the Godfather),

movie could start opposite as the ending of the first, the power of the Corleone's on the decline, and the elimination of the Corleone topmembers, Michael Corleone being shot, Vincent Mancini, Anthony Corlone, the capo's,

the "ennemy's" (powerfull south american drug cartel with connections in the US and sicilian maffia, and CIA connections) would have set their eyes on the legitime assets of the Corleone's (banks, real estate companies) they want to use to whitewash their drugs money,

the "heir" of Michael would then come forth (in flashbacks we see how Michale 'trained' him), taking head of the family, Anthony and Vincent would now him adult children also (could make a love story with one of the daughters and the "heir")

Michael Corleone, would just as father before survive to his wounds and be the consiglieri his father was to him, fulfilling the Corleone revenge, before returning to Sicily where we see him die as the end of the 3rd, the "heir" of the Corleone's could then turn the Corleone's (the remaining off course) finally on the legit path and eliminating every remaining treath to the Corleone's, and becoming himself the next big Don,

the time period would off course be the 70/80's

well that would be how I could live with a sequel