If I had to bet, I’d put my money on epub. The model of providing a zip file of HTML with an XML manifest is an easy evolutionary step which has been invented independently many times. CMX/Cocktail could easily migrate in that direction.
Epub uses an XML manifest, not something developed under the microformats.org umbrella. My experience with the XML is that it’s a bit cumbersome and could be simplified by using more plain old semantic HTML in its place. But whether a microformat.org production is more useful than plain old XML is a different question.