The "interesting" kids for GF3 would have been Solozzo's children, come back for vendetta against Michael. It would have made for a heck of an action film.
:-)
Seriously, I don't really know what any GF3 could add to the story. GF was a full tragedy. It unfolded in GF2 by comparing the lives of Vito and Michael and by showing how Michael isolated himself more and more until he became a lonely man who lost everything, first of all his family. And the family was the reason why he joined the crime business.
Of course, we're Godfather fanatics, we'd even watch a Godfather TV series, as long as Al Pacino played Michael Corleone. We'd watch a prequel, etc.
GF3 adds some political stuff to the story. But I can get that from the newspapers too. And yes, he loses his daughter physically. But he had lost already every-one at the end of GF2.
