We like that the Problems can send a message to all related Incidents, but would also like the Problem to be able to send a Notification to the Requesters. Perhaps use the same process of the Private or Public button used in Incidents while doing so.
Our current use case for this would be a Problem that we had which caused an issue for multiple users and generated a number of Incidents. The solution was an upgrade that we had to roll out. Once the upgrade was rolled out it would have been great for the owner of the Problem to be able to put out a message to all the people who logged the incidents that the Problem was resolved and to test again.
The Owners of the incidents could then close them once the users got back and confirmed the issue was resolved. Currently the Owners would need to reach out on each of the Incidents manually to notify the users the issue was resolved and to test the solution.