Who exactly is Madison Pettis? Well, as much as I hate for someone to live in the shadow of their father, that might be something that Madison has to overcome. She is Dwayne "The Rock" Johnson's daughter. There is no bigger fatherly shadow than the one created by "The Rock." He's a huge action star, if not the HUGEST action star, which is exactly why Madison Pettis might be perfect to cast in an action movie. She has legendary genes, after all.
Her name was in the bucket despite her young age because at one point producers had the idea to cast a young Wonder Woman so that she could grow in front of audiences like in 'Boyhood.' It could have been a cool idea, but the Wonder Woman property is far too important to put in the hands of such an experimental idea.