It would be very helpful if such a feature can be acheived. I always have difficulty when visiting a long-time not visited org file, where it may have lots of headings and their sub-headings, and so on. I need to have an overall grasp of which headings have sub-headings when collapsed and when only first-level headings are shown. Since, headings and sub-headings are a lot, so I'm used to only showing first-level headings and opening up a certain heading when I know exactly that it has sub-headings. If I use C-Tab, all second-level sub-headings of first-headings will be shown up, which adds more obstacle for navigating!
In ideal, sub-headings of their parent heading should also have this symbol in the end of them, if they have their own sub-headings.
It's gonna be a dream to come into reality, so I can finally, completely move from zim to emacs. Among Gui note-taking programs, zim is the most powerful yet simple that has every feature that I need.
But the only downside of it is that it relies so much on mouse, instead of providing actions that can completely be done on only keyboard. When working on a laptop, which provides less-comfort for controlling the cursor, this issue has become more serious for me.
For example: In org-mode of emacs
* First-level heading that has sub-headings...>
** Sub-heading1 that has its own sub-headings...>
*** Heading1...
Just some text
*** Heading2...>
Some text
**** Heading1...
Just some text
** Sub-heading 2...
Just some text
Or any plugins that do exactly the job as a sidebar as what Zim does

