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