Gingko is quite a nice app! Some things which would make it a must-have (for me, anyway) are:
Versioning per card, with the ability to expand a card to see a stack of versions of each card, or the diffs from one version to the next, etc.
Embedding one card in another, which would mean it becomes possible to have many trains of thought, but to gather them up and project them into a "final" column that represents a summary of all of the most important notes among cards in different branches.
Linking between cards, which would mean being able to easily cross-reference notes - sort of like a personal wiki, I guess.
Gingko is quite a nice app! Some things which would make it a must-have (for me, anyway) are:
Good luck!