I think almost any proposal for a GF IV would feature Pacino, Keaton and Shire as Michael, Kay and Connie, in at least some limited roles.

The attraction of a GF IV, if done well, would be in the back story - coming up with credible actors to play a younger Vito, and younger versions of Michael, Sonny, Fredo and Tom, as well as other characters like Tessio, Clemenza, Pentangeli and Roth in their younger days.

But any version of GF IV would almost have to touch upon the final 18-20 years of Michael's life and what happens to the Corleone family during that time frame.