Independently task progress.
It would be great if multiple persons a task is assigned to could independently indicate progress.
I would add:
As of today, when several users have the same task, there is no mean to see who has completed the task and who hasn't.
Today, when one assigned user to that task select "completed", Planner considers that all the users have completed it and show the task as completed.
The task should not be shown as completed before ALL users have clicked on "completed". Individual progress should been shown in the task.