Rebecca Hall, known for her work in Vicky Cristina Barcelona, The Prestige, and Christine, was supposed to be the first main female villain in a Marvel movie. But the president of Marvel Studios at the time, Ike Perlmutter, didn’t think they’d make as much money in toy sales:
“There was an early draft of Iron Man 3 where we had an inkling of a problem. Which is that we had a female character who was the villain in the draft. We had finished the script, and we were given a no-holds-barred memo saying that cannot stand and we’ve changed our minds because, after consulting, we’ve decided that toy won’t sell as well if it’s a female. So, we had to change the entire script because of toy making.” —Shane Black, Uproxx
If you rewatch the movie, it becomes much more obvious that Hall’s character was simply split into two (Guy Pearce’s Aldrich Killian became the main villain). Hall’s character invents the technology that Pearce’s character abuses, with Hall eventually dying. It’s easy to imagine a version of the script in which those are a single character, with the inventor of the technology simply being the villain.