One of the main differences between comic books and comic book movies is the ability to do certain things with certain characters in certain situations, and then relay them in a certain amount of time. Movies always have a constant state of motion, even when sequels are a given, cause eventually the movie will end. Whereas, for a comic book, the arc can go on for a while without break or hesitation.
Sometimes an iconic role is made by someone who does it once so masterfully that you think it cannot be done again. Then, there are roles that happen over many movies, and in some cases decades, and thus when you hear that characters name, you think of that person. For Ian McKellen, you can honestly say he’s done that latter one twice. Once for Gandalf in the Lord of the Rings Saga (both LOTR trilogy and Hobbit Trilogy) and as Magneto in the X-Men saga. In fact, his Magneto has appeared in all but two X-Men movies (Origins and First Class). When it was announced that he was coming back for Days of Future Past alongside other X-Men veterans, many wondered if he would show up (somehow) again in a future X-Men film. According to McKellen, the answer is no; at least for now. McKellan recently stated to Digital Spy: