I would like to see in an outliner what Brett Terpstra states in his blog when he wrote about the Tree app;

“An outliner is an outliner, but Tree adds one thing that makes it infinitely more useful to me and my non-linear thought process: a horizontal view of the outline. It’s basically a right-aligned mind map. Seeing each level of the hierarchy in line with other siblings across different branches suddenly brings the whole brainstorming thing together for me.”

https://brettterpstra.com/2012/04/02/app-review-tree-an-outliner-i-can-love/

There was a further discussion about this in these forums:

https://www.outlinersoftware.com/topics/viewt/9131

Maurice Parker wrote:

I've just started a project to make a simple, clean outliner for Apple
platforms. It is called Zavala and is in the very early stages of
development.

I'm looking for users to provide feedback on what they would like to see
in an outliner. If you've ever wished that you had a direct line to the
developer of an outliner before it got released, this is your chance.
I'm very interested in how people are using outliners and what features
they need in the real world on a daily basis.

If you have macOS Big Sur, there is a developer release available to
kick around. You wouldn't want to trust any important data to it,
because it is under active development and might scramble your stuff
completely. You can take a look and provide feedback as to what works
and doesn't work for you. Even if you can't run the developer release,
there is a forum for the project there. I'd still like to hear what you
think makes a good outliner.

The project is open source and Zavala will always be free. I hope some
of you join me in making an outliner that is a joy to use.

https://github.com/vincode-io/Zavala