You could try Little Outliner[1]. Dave Winer has been making outliner software for decades[2]. I remember playing about with a desktop app years ago. I guess Fargo is the latest iteration.

[1]: http://littleoutliner.com/ [2]: https://en.wikipedia.org/wiki/Dave_Winer#Career

Someone else on this thread suggested concorde, which seems to be the engine behind fargo.

[1]: https://news.ycombinator.com/item?id=7512666