What do you think of adding comments to files and/or directories? For example, I'm watching a video related to GNU/Linux and I wanted to comment it as such "This video is based on RedHat" then I could see the comment on the tooltip or the properties menu. Another good idea (I think) would be adding a bright star * to the upper-right corner of the icon indicating the file/directory has a comment.
Thunar is a simple file manager, and as such it does not make much sense for it to support this.