Two feature requests:
Display a breadcrumb at the top of the window, so the user can see exactly where he is in the hierarchy and instantly click back up to higher levels. Clicking on a breadcrumb item would navigate to that item in the hierarchy and focus the item. It would essentially have the same behavior as the sidebar, except it would allow for navigating to any item type, not just projects. In my view, this is the most important feature currently missing from TaskPaper.
Allow the user to assign icons to tags. For instance, my outline is littered with @today tags everywhere. It would be much cleaner-looking and less distracting to have TaskPaper automatically replace each instance of @today with a star icon or something. Naturally, this would only affect the display within TaskPaper and would not alter the underlying text file, which would still contain the @today tags.