By not having as many characters that need an equal share of screen time and development as The Avengers does. Yes I am, and it's not ridiculous. Because not only would he be a brand new character, he'd be a brand new character who would have to be a central focus of the plot, since his entire character arc would be tied up in the main conflict of the plot. That's not the same thing as War Machine or Falcon. Having the creator of Ultron be someone the audience is already familiar with would streamline the plot tremendously. And in a movie with this many characters already, a streamlined plot os an important thing.