I couldn't imagine anyone being able to bring the combination of brash egotism, genius, and glorious childishness that is Tony Stark to life the way that Downey can. The years of painful self destruction have given enough depth to Downey's own character to allow him to be bold, zany, and yet totally human. He makes a comic book character come to life with a humanity and a realness that "superhero" movies of the past have struggled to create. The Marvel universe has come to life, and spawned a slew of blockbusters entirely based on making superheroes believable, approachable and human. And without Downey's complete empathy with Tony Stark, I don't believe that this could have happened.
So, who could take on the responsibility for continuing to represent superheroes as living, breathing, complicated people? How about the guy that made pirates a thing? Outside of playing the foil to Peter Pan, how many movies have tried to depict pirates in a highly stylized and yet believable way? Most people dismiss Depp's characterization of Captain Jack Sparrow as a child's character. They see the role as another Disney bid to sell rides and toys (as it surely was). But no one could imagine the level that Depp's Captain Jack Sparrow would resonate with the public, or how iconic that his character would become. Just as Downey made Ironman "not just another action flick", Depp made Pirates of the Caribbean into a blockbuster fantasy world that captured the imagination of both adults and children alike.
Both Depp and Downey have the capacity to love and respect their characters so fully that they can convince the audience to love them too. Both can take characters that other actors would not take seriously and showcase them as fully human, forgeing a relationship between the audience and the character that lasts long after the lights come back up and the curtain falls. The magic that these two men bring to the big screen is the magic of believable fantasy. They send us stuffy adults back to the days of our childhood when we could dream of being superheroes and pirates and we fall asleep living adventures as we did when we were young.