Great idea, @JamesT. Here’s my list.
1. Multiple parents filter
My system of organization is somewhat two-dimensional. I would greatly benefit from the possibility to filter the noteboard so that only cards that share a list of specified parents are shown.
2. Batch editing
Currently, once you committed yourself to a system of organization, it is unfeasible to alter it due to the lack of batch editing, the ability to edit meta-data of multiple cards at once. I also mention tag-renaming capabilities here.
3. A true inbox
It is a very common concept for apps to have a dedicated space to drop new input when no further information is given. This may become particulary important once you can share to supernotes using the android/ios share menu. While the thoughts section is intended to serve the identification of to-be-organized notes, for me, a simple inbox would serve that purpose better, as it would allow for handling of new notes using an inbox-zero-strategy, which I failed to apply for the thoughts collection.