You can deceive the user pretty easily with content that looks like the main page and prompts them to do something you want them to do. (like login to the parent app even though they are logging into your app, anti-phishing stuff won't take effect if you keep it all in the iframe)
You could also just execute whatever JS you want if there is a browser vulnerability you want to take advantage of.
You could also just execute whatever JS you want if there is a browser vulnerability you want to take advantage of.