Jodie Foster as Princess Leia? I think not. Carrie Fisher seemed to be made for the part and brought it to life on the big screen. And Harrison Ford IS Han Solo, just as he IS Indiana Jones. Anyone else but Tom Hanks in the role of Forrest Gump just wouldn't cut it. That is not to say that the other actors who were considered for role are not GOOD actors, they are, but Bruce Willis IS John MacLain and just doesn't strike me in the Sam Wheat role which surely was meant for Patrick Swayze. Good actors MAKE the role.
To prove my point, there are certain characters that no other actor can emulate and no matter how hard they try, producers/directors/writers just cannot remake TV shows/old movies that are classics into today's medium. IE: Starksy&Hutch / Bewitched / The Beverly Hillibillies to name but a few.
It would make for interesting viewing indeed if we could "see" those actors who turned down the above mentioned movie roles actually in those same roles. I doubt very much whether the end result would be the same!