I don't see any reason to remake the Godfather. The only films that I think are worth remaking are either those that are potentially great but let down by pretty stupid flaws, or those that are so badly dated that a modern version would improve them. Films like White Heat and The Public Enemy spring to mind.
For me, The Godfather is about as perfect as a film can get. So there is no room for improvement there. And as for it's age, I would say that generally films were much better in the 70s than they are today, so a modern approach to it wouldn't neccesarily be able to improve it. Plus the fact that Pacino, Duvall, Caan and Brando are among the best actors ever, and to replace them with an equally talented cast would be nigh on impossible.