I think it should be possible to run A/B testing on emails in the same way that the current landing page A/B testing works. Basically, create the ability to convert an email asset to a test group, and when sent by a smart campaign, send version A to a fraction of the list, version B to a fraction, and so on.
I know AB testing is possible using the Random Sample function in a smart campaign, or by manually segmenting the list through smart lists, but this isn't terribly user friendly, and can be time consuming. As a user who likes tests everything, the addition of this feature would be a huge timesaver for me, and would likely make it a whole lot easier to track results later.
.png)