TreeSheets (native cross-platform app based on wxWidgets) is great and in need of new contributors.

For non-linear organizing on iOS, the indie app MindScope (https://www.macstories.net/reviews/mindscope-review/) is uniquely fast for brainstorming and re-factoring visual layout, before exporting an outline to another tool. A single node can be linked from more than one location in the hierarchy.