Notion
A highly-flexible, cloud-based app that individuals and teams can use to write, plan, collaborate, and get organized.
Planning
MindNode
Allows you to create visual outlines of your ideas and responsibilities. Useful when designing your system and for planning projects.
OmniOutliner
OmniOutliner is a natural complement to OmniFocus. Among other things, it can be used to plan projects and as a repository for project support material.