Great idea for a thread Hondo! The only one I can think of that could really match Duke in the longterm would probably be Clint Eastwood, he is somewhat of an icon having made many great pictures over a near 60 year career and he also benefits from much exposure to younger and wider audiences by having his films played all the time on television. Another contender could perhaps be Stallone because of longevity, appeal and again his films are never off television. Besides that I just don't see anyone coming close to the Duke in worldwide fame and lasting appeal. While Harrison Ford has been good in a few things and iconic in Star Wars I don't think he's famous or prominent enough to match Duke's appeal worldwide, in 50 years people will probably still be watching Star Wars but I think that will be more about the film than Ford himself whereas John Wayne movies are almost a genre of their own.
Forgot about Clint Eastwood. He is the closest icon to John Wayne. I will concede to you on that one. And of course the Indiana Jones Movies too from Harrison Ford.
Cheers Hondo