I think it would be really cool if you were able to setup A/B tests so that Marketo could automatically identify a winner and default to the winner without a user having to make the change manually.
For example, when you setup an A/B test for two landing pages, you could add a rule that says "whatever landing page is converting better after 200 visits automatically send all leads to that page"
This would be awesome in drip campaigns especially, where if you setup tests for many variables throughout various emails and landing pages in the drip (subject lines, copy, design elements, forms, etc.), overtime, your drips would automatically collect data, build intelligence, and become smarter and smarter withou you having to go back and make manual changes.