Praxeas,
I tend to agree with you.
I am for taking a storyline from within the story of the GF and not call it a sequel. I have always been interested in Vito's rise to power in the early days, focusing on his family and how he began his life of crime, or perhaps focus on another of the families.
I think, however, all new actors would be required (sorry to say),
because all of the original main stars can not play the younger parts anymore. That's where the difficulty comes in, however, because Pacino/DeNiro/Brando would be hard to replace.
TIS