Provide a means to specify video aspect and menu & audio language.
Search whole of history rather than just previous page when deciding whether a link goes back or forward.
Set Mozilla's idea of the pixel aspect ratio to match video pixels. (For extra credit, scale preview mode to correct for the differing aspect ratio.)
Add keyboard command for "top menu" in preview mode.
Add support for videos in preview mode.
Support more than 99 videos by grouping them into titlesets.
Avoid exceeding the maximum 128 instructions per PGC.
Allow explicit ordering of videos.
May need to override default menu navigation actions because dvdauthor seems to cause rather undesirable behaviour when buttons are not arranged in a grid.
For any page with no links, add a "back" link (this raises the issue of localisation, of course).
If a page is too large, split it and add "next" and "previous" links (seems very difficult).