From my personal experience with other mindmap tools like Xmind, iThoughts & MindJet, I can tell you that the option to add large amount of text + media files ( like the full-featured whimsical documents ) to the mindmap nodes is a must.
The mindmap tool in whimsical is great, especially because of it’s performance and the opportunity to mix it with other things on the infinity whiteboard, but please add this important feature.
Perhaps with the option to toggle between the mindmap and an outline view.