We flirted with a super-ajaxy theme a long time ago. While a lot of the good ideas it had didn't make the cut, one thing that we might be able to due, is simply have the top-level page content be "full page" (e.g. width=100%), and then allow the site-tree area to be expanded by the user (it would have more size on many people's monitors anyway if the pages are 'reflowed' in this way). The trick will be to find a decent CSS theme as the base that doesn't break IE 6, and doesn't take forever to load.

The sidebar can be a fixed width in IE 6 if needed to simplify the experience, as long as other browsers can benefit.