Like in OneNote. Make it possible to add tags (both symbols and words would be great) to individual lines, and then have search folders that collect items with certain tags.
I was searching for an offline alternative for Checkvist and found you. Tags are definitely one of the most important features needed for using UV outliner as a GTD tool
I like this idea. I could "tag" different story threads in a novel and work on each plotline separately if I wanted. Or I could tag all scenes involving a certain character, setting, etc. For a novelist the possibilities are endless.
I think the cloning feature also suggested on this forum would support this and be more powerful. To tag an item, you would clone the item into a row ("folder") of your choosing. The advantage tagging has is that you can just type the name of the row you want to move the current row under rather than dragging it, but it could be possible for cloning to include a "quick-clone" feature that does this too by letting you search for the row to move the new clone under.