For accounts that have multiple Users (specifically Client Users), it would be great if they could add or remove themselves to receive notifications on specific threads.
For example, let's say an account has 2 Users (User A and User B).
User A has Inbox Notifications turned on to receive all notifications.
User B has Inbox Notifications turned off. But, User A can add User B to a specific thread to receive notifications (and User B could also add themselves to the thread).
Essentially this would work similar to how ticketing / project management tools work when you want to notify a teammate.
Another variation could be "internal"/private messages on a thread to notify a teammate (or other User).
Attached is an example of how our Project Management tool functions for something similar.