I think Scott Caan has too much of the baby-face look - a young Sonny has to look hardened and street tough - and as far as acting credentials are concerned, Ben Affleck looks like Sir Laurence Olivier compared to Scott Caan at this point of his career.

But who knows, suppose Vincent and Grace Madison got married shortly after Mary's death in 1980 and had a son? (a-la Michael and Kay.)

That son would be in his late teens when the coda sequences might well take place (late 1990s/early 2000s.)

So maybe Scott Caan (with darkened hair) could play Sonny's grandson, who might be getting involved in the underworld in 2003-2004???