Spoiler ahead:

I haven't seen the whole coda, but watched a youtube clip of the ending. It seems to me that the Sicilian slide at the end is not a productive choice. I understand that, because of age and expense, they couldn't shoot a whole new ending.

There's something in the original novel about how Michael's Sicilian nature is awakened by his time in Sicily, but I think that Coppola's story is, at bottom, an American story, an immigrant family making it in America with the twist that the family business is on the wrong side of the law. I've always thought that Kay's "Sicilian thing" line was strange because she couldn't specify what the thing was.

Also, I think Sicily has ceased to be exotic in the 50 years since the first movie came out. A lot of that has come from the mafia movies that followed. A lot more because we're exposed to all cultures now, and it turns out that a lot of them have lawless sides. To me, it doesn't seem like invoking "Sicily" is bringing anything unique to mind anymore.

Edit: I've also been wondering if the slide might have been something Coppola had rolling around in his head for the ending to GF2. It fits there about as well, and they did take the trouble to explain what "Cent'anni" meant at the beginning of the movie.