Thinking of a Godfather without Pacino seems unholy..
So then i guess its time to be unholy!

Talia Shire is an excellent actress who really shines when she is surrounded by other great actors. The chemistry between she and Al in the trilogy is one of the great on screen pairings in movies history. Garcia isn't enough to encourage a knock out performance from her though.

Andy Garcia can be both overrated and underrated. Underrated in that he was fabulous if Part III and there are those who forget that and overrated by people who say "oh he's an Oscar nominee' and forget his movies since have bit big time.

All that said, i think it underscores he fact that we would need to do the flashback thing. I truly believe that Deniro would be game (if he agreed to Analyze That he should embrace this with open arms!) And in these flashback scenes PLEASE hire people who will knock your socks off, unknowns or otherwise.
So i would keep:
80's Era:
Talia Shire
Andy Garcia
Bridget Fonda (who CAN act)
Nicholas Cage in some form (if your gonna go to the well, get someone GOOD
Franc D'ambrosio (for an Opera-esque scene)
and even, don't kill me
Diane Lane and Madonna-Diane could be the new Kaye type and Madonna the new Connie. I think it would be a nice Juxtaposition