Yeah, if a GFIV is made, presumably covering the time period 1927-1941 which hasn't yet been touched on in the movies, seeing Fredo as at least a somewhat competent guy in his young adulthood would make sense. But somewhere along the line, he screws some things up, and his fate is sealed... from then on, Vito, and Sonny, and Tom, and Michael all know, that Fredo can't handle really being part of the family operations...