Puzo and FFC were working on a new one in the late 90's but then Puzo died and it went to rest, probably permanently. They were toying with the idea of having a Vincent/Sonny storyline and flashback. Leo DiCaprio was rumored to be up for the part of young Sonny.

Anyway, search the boards for info about it. There is a lot.

When the books came out (Returns and Revenge) I think the studio heads were hoping they'd be worthy to turn into movies but they aren't. I don't see that happening.