I'd love to see full-length movies and tv series episodes on the Marketplace. Beavis and Butthead, South Park, Talladega Nights, I have these, and it'd be legal to put them onto my Zune since I already bought them, but there's no simple, legal way to get it from the DVD to the Zune, let alone the computer.
Also, a media-sharing system between the 360 and Zune'd be great. For instance, I pay for and download a movie on my 360, there's nothing I can do from there. I should be able to put it onto my computer, Zune, burn a DVD, or whatever! Make it happen, Microsoft!