Would be great if we could add custom fields / attributes / metadata for selected elements.
For example, if you have a portfolio matrix or a project management board, you might just add the picture and title of the project/product to the board, but when selecting each item, you could add notes to it, some custom fields (like Asana) on timing, cost, category, deadline, โฆ or any kind of note.
This would also be awesome to filter what is visible (or dimmed) on screen by any preset custom field.
Creately (Features | Creately) already has this kind of feature - which is mine opinion is a great advantage over Miro, but is implemented in a very clunky way.