The Friends and Foes system is very poor, and we all know!
It would be great to add some new features to it.
It would be great if we could receive notifications of our friends activities, if we enable that notification.
And there are certainly others great things that phpbb could have with a friend activity system.
Other thing that would be awesome if we could create groups for PMs, this would improve the PM system. Because we would have custom groups made by us, so we can send a PM to a specific group of users made by us. And not a official group with permissions.
It would be like a list of users that we send a lot of PMs.