The poor-girl turned princess story is always a success with books and movies. That is basically the story line behind My Fair Lady, directed in 1964 by George Cukor, based on Alan J. Lerner and Frederick Loewe's 1956 play, and Pygmalion, directed in 1938 by Anthony Asquith and Leslie Howard and based on George Bernard Shaw's 1913 play. Professor Higgins (played by Rex Harrison and Leslie Howard) takes Eliza Doolittle (played by Audrey Hepburn and Wendy Hiller) off of the streets and transforms her into a true lady that can pass in the high society of London. While the transformation was a total success in the above cases, I wonder what would have happened if she had been a failure. Would Professor Higgins still have wanted her to remain in the house? I imagine that he would not want her around the house; but, in order to appeal to the audience, the story would still have to end with Eliza and Higgins together.
Professor Higgins worked Eliza around the clock; he would have been ashamed if she had failed him. He showed this characteristic in the plays and the film versions. In My Fair Lady, he even had a song which was devoted to show how much he did not want a woman in his life. Imagine if she had made it all the way to the ball with Higgins believing that she was perfect, and then she had crashed. If this had happened, I think that he would not have dealt with that well. I think that he would have told the entire party about the experiment with Eliza, and she would have been so embarrassed that she would run out of the ball.
If I were to rewrite the ending of the play so that Eliza was to fail at the ball, I would have Eliza run out of the party. Then after a few days, Higgins would begin to miss Eliza, and he would find her and ask for her forgiveness (in his somewhat rude manner). After she would go back to him, he would turn her into a real lady instead of just a fake. This would still make a great ending for a movie or play that could be sellable to an audience. I would want an actress in her mid-twenties that had a believable English accent to play Eliza and an actor in his late-thirties to play Higgins.