If they were to do the two seperate storylines then something like this

Modern Day
Vincent Corleone - Andy Garcia
Michael Corleone - Al Pacino (Perfect age to play a retired don who gives advice)
Kay Adams - Diane Keaton
Connie Corleone - Talia Shire

Anthony could have a larger role and could be played by a number of characters depending on what role he has. Another would be another of Tom Hagen's as Consolieri.

Past

Here Sonny, Tom Hagen and Vito Corleone would have the key roles. Fredo could also play a slightly larger role in the film and I think that it is here where Milo Ventmiiglia should be used.

Santino Corleone - Leonardo DiCaprio
Tom Hagen - Christian Bale
Vito Corleone - Robert De Niro (Similar to Pacino in modern one)

I would have either no Michael Corleone or very little of him in the past one as it'd be hard to centre it on Sonny and still have Michael so this one could be set when Michael was in the army. To keep it linked though you could have Vito and Tom talking about how disappointed they are etc.

De Niro and especially Pacino would have more minor roles in the films.