The novel describes Sollozzo and Brasi as men who radiate danger. The cast for these characters was perfect, I think.

I think that GF2 was a great film, but it lacked dangerous characters.
Roth? Too old.
Neri? We don't see anything really threatening of him. He washes his hands after the murder of the prostitute and (in the delted scene) slaps the owner of the casino.
Mike's bodyguard? I wouldn't want to meet im in the street at night, but no comparison to Brasi. Too soft and old.
Fanucci? Forget it.

And in GF3, every opponent is old. Mosca's nephew (or was it his son?) - ridiculous. The twins look like opera characters.