My #1 addition would be the ability to tag items with a value, and then be able to see all tagged items in a separate view that allows sorting by the tags. One use of the ability would be to assign priorities and dates to action items, and then see a listed sorted by priority and date without disturbing the underlying outline. This was Grandview’s primary strength. Circus Ponies Notebook allowed a limited version of it. OmniOutliner started on that path, but then the capability got pulled into OmniFocus.
For me there’s too much overhead in having to use a separate piece of software for handling action items when all of my planning and information storage is already being done in an outliner. As is OmniOutliner does let me maintain an automatically-sorted prioritized list of items that’s easy to copy/paste from/to my primary notes, so it gets me far enough along that path to do what I need–just not as efficiently as I would like.