You can also throw assigned into the mix. When a message or topic no longer needs follow up we unassign and archive it. Works quite well.
We do also use Tickets Plugin 🎟 by @angus which facilitates assigning tickets and also provides ticket tags for priority, status and reason. Definitely worth a look.
But the tickets plugin still needs some work on the dashboard for managing tickets. So we use it to set up tickets but rely on the assigned list to manage the backlog of tickets and work through them to resolution.