I would vomit if I saw Colin Farrell take any Corleone role, especially Michael's. The guy is a disgrace. Cute, yes, but a disgrace in every other way.
All of the suggestions made on this topic are sincere enough attempts, but I think THIS is why there never should be a GF4. Why mess with something perfect? Oh wait, that already happened with GF3. Ok, then, why mess further with something that used to be perfect?