I always wondered would MS do something like this. Make a new Zune based off WP7 which can run all the same apps.
I'd consider it *very* unlikely at this point given how much the MP3 player market has been in decline. A Windows Phone is more or less a Zune with a phone in it in so many ways anyways. They would be far better off just bringing any missing Zune features to Windows Phone 8.