Leo is fairly cool. Been around forever, but not many people know about it.
I'm a heavy Emacs and Org mode user. But at this time I've given up on being proficient in Emacs-Lisp and how it ties to the whole Emacs ecosystem.
So I searched for a self-extensible editor in Python, and find Leo.
I haven't taken the time to learn it really well, but I did fiddle with some tree editing in Python with it, and it works as advertised. If I didn't have to work for a living, I'd spend most of my time porting over the cool aspects of Emacs to Leo.
Unfortunately, the documentation/web site is very opaque. Not so bad that it's useless, but bad enough that if anyone wants to learn it well they'll have to do a lot of Google searching (in the mailing list) or code browsing.
Also, to be frank, it's not a great editor compared to Vim/Emacs. But that should be easily fixable with scripting/code changes.
http://leoeditor.com/
Leo is of particular interest because it automatically syncs between the tree and code files: http://leoeditor.com/tutorial-programming.html
The approach is documented here: http://leoeditor.com/appendices.html#the-mulder-ream-update-...