Much like Scarface, I can't picture another actor playing Rambo....may not have to, though since Stallone is still making Rambo movies. I think the most interesting thing that could be done with either character happened in the original movies. Tony's rise was really in America, anything prior to that would likely pale in comparison. Same with Rambo, I don't think you could do much other than have a young Rambo in Vietnam which wouldn't be terribly interesting to me, or Rambo getting harassed in previous towns prior to First Blood, which also wouldn't be all that interesting since we know that he didn't lose it until First Blood. Would have been interested to have seen First Blood more like the book, except it's obvious why they didn't. Rarely can you make something as dark as most books and expect it to sell as a movie.
I'm not there with a Joker-centered prequel. Not knowing where he came from makes him that much more interesting.
Meanwhile, I'd want to see a prequel to Up. Carl and Elly's relationship was summarized in five minutes and yet it was the crux of of one my favorite movies ever. I'd like to see a slice of it out of montage form.