Milestone Active button
It would be great if assigned co-workers can see that someone is already working on a milestone. Thus preventing two people starting the same milestone.
Most Milestones in our firm can only be completed by one single person. With this option the milestones tab can become a place where a team of co-workers can pick up their tasks. Which they could mark as "Active".
Added bonus : Co-workers can choose the tasks they will be working on.
For larger teams i would imagine an "Activity overview" for group managers, where the milestones per employee are shown.