Hi, long time lurker, first time poster. Interesting casting ideas, all around, but I've got a few of my own...

If the producers decided to make a GF IV film with a similar story structure to that of Part II, they would have to go with (mainly) lesser known actors as the younger versions of the original characters in the flashback sequences. Here, then, are my ideas:

SONNY CORLEONE: Aaron Johnson, the kid from "Kick-Ass". Yeah, I know he was a real nerd in that, but Google him, he looks an AWFUL lot like a young James Caan, right down to the same smirk and curly hair.

LUCA BRASI: Bobby Canavale, most recently seen in the current season of Boardwalk Empire. This guy IS a young Luca Brasi!

TOM HAGEN: Aaron Staton, from MadMen. Same cold eyes as Hagen, a close ringer for a young Robert DuVall, could easily pull it off.

DON VITO CORLEONE: This was a tough one, all around. I would've liked to have gone with Robert de Niro, but let's face it, he's too old now (and I hate the idea of applying CGI or make-up to "youthen" him). Physically, the best choice for this would be Billy Zane, who currently looks like a transition between young de Niro Vito and old Brando Vito -- I'm just not certain he's got the acting chops to pull this role off anymore, though he has given a few good performances in his time (and Brando was considered a risk when they filmed the first Godfather, so it might not be THAT much of a stretch...)

Anyway, those are just my thoughts.