FFC and Puzo have always based the GF on real life. Today, the mafia is severely weakened or doesn't exist at all. I think to reflect real life, the Corleone family would have to take on the likeness of the Gotti crime family (or a similar one) with back stabbings and gov't informants to bring down the family.

I also think that is much more interesting than seeing them rise to power again.