I know we have widgets for sidebars and we've now get very customisable templates in the form of Dixie, Nelo and Triden but how practical would it be to make a template where everything is a widget? If you've played round with My Yahoo! with its widget based front page or iGoogle then you'll know what I'm talking about.
The idea of having a template where I can drag and drop my page layout sort of appeals.
But having said that I suspect the current way WPMU (and WP) handle their templates with different structures for different parts of the layout (index, page, single, etc) would make it pretty hard to do.