The ability to be able to assign an existing comment/attachment from the mother task to a sub-task, or change it’s assignment to a different sub-task.
In Asana it is too easy to add comments/attachments to the mother task and forget to put them in subtasks. This mess is hard to sort out - or impossible - without the ability to reassign existing content. Drag and drop would be elegant. Reassign to different task/subtask in the dropdown would be fine