Ability to "Send" an item or multiple "checked" items to a top or tagged item to appear under.
CarbonFin is the closest outliner I have found that compares to the wonderful classic Grandview outliner program. (Grandview was sold decades ago to Symantec (the security company) who promptly abandoned it (sigh). As a television writer I used Grandview all the time to outline my scripts. One feature it had was the ability to "Send" an item to appear indented under another item with just a keyboard shortcut. Grandview had many other features, but I'd settle for Carbonfin having this feature, Here's a link to a Google book summary of Grandview's features: https://books.google.com/books?id=Eq0wALnyM_MC&q=%22grandview%22+%22friend%22+outliner&pg=PA34#v=snippet&q=%22grandview%22%20%22friend%22%20outliner&f=false