I'm not sure how difficult this would be, but I'd like to have the ability to insert my own comments, observations or even notes in an actual L4 document.
So if you're reading a book and something tweeks your attention you could insert your comment etc., at that very point and then continue reading. There was a very nice product called iMarkup that gave one the ability to do this on a web page, but it has since been cancelled from production.
Not sure how difficult that would be but I think it would be nice tool to use.