The web version is too small and not practical to use on an Android phone. Can you either make an Android App or make a mobile version of the website that adjusts to the users device.
There are now more Android users in the world than iOS. Please take our money.

Allow ability to group outlines on main page. Group by topic (work, personal) or by project.

Mindmap / PDF / Markdown ...

I want you to wider the width of the editing area.
I want you to variable to match the width of the web browser viewing.

An autosave feature

If you have more than a few outlines, you'll really need to group them into folders. CarbonFin Outliner doesn't currently support folders in Dropbox.

So the storage location defaults to Dropbox.

It would be nice if one could add priorities to tasks and sort tasks by priority.

Add a radio button to options, or modify Archive, to protect an outline from being inadvertently deleted. This happened to me when I was trying to deconflict a file on my iPad with one on my iPhone. I inadvertently swiped a similarly named outline and the sync process deleted it instantly on both devices with no way for me to undo that deletion. I lost an outline representing the entirety of my work in the month of June! With a protect mode, it would at least give me a question for verification.

support for copy and paste in the desk version, just like in the ipad version.

Export/import to/from plaintext with basic Markdown formatting would be really nice. With "basic" I mean: no text styles, no checkboxes, just the sibling/child structure (and maybe support for notes).

This way we could do major editing with any text editor on the Mac/PC.

As it is now reducing font size ("small font") doesn't reduce line height. This is not terribly useful.

With "small font" also line height should be reduced (and accordingly the size of the triangle and checkbox).

I would prefer the following format: I.1.A.i.a. etc vs the current legal format.

I use master lists (eg for food shopping). I check the items I need then uncheck them when bought. A filter to show only checked items will reduce as I buy items but my master food list is retained.