Luca Brasi is not the focus of the story, nor could he ever be. There is a reason he played only a minor role in both the novel AND the film. A character like that is very compelling, very interesting as a supporting role. To drum up a Luca Brasi story just for the sake of getting a GFIV off the ground it something akin to grasping for straws.

And that's just exactly what ANY attempt at a GFIV would be...grasping for straws.

Paper straws. The kind like when I was a kid...that would shrivel and crumble to shreds the minute they were used.

Apple