I'd like to add my 2 cents.
Of course the popularity of classical forum systems are in steadily decline - and it is unavoidable. There are now so many different ways to communicate with almost instant access and quite comfortable mobile apps. I do not even try to enumerate them, but the list will start with whatsup, twitter, facebook, slack, discord, etc..
On the other hand all this instant messaging systems are pure junk. These apps are great as a way of instant pushing of messages, but in a long run the content of these messages is without any real value. From this point of view the content gathered in classical forum systems is a completely different league. I hope you know what I mean. From this point of view phpbb still have it's important place/niche in the ecosystem.
But there is a real competition. From my point of view there two/three types of topics - (A) (kind of) announcements with little discussion below, (B) question-answer, (C) general discussion (like this topic).
For (C) phpbb is great, but this is a minority of topics.
For (B) the king is stackoverflow and its children.
For (A) people often prefer wordpress style systems.
So if you ask for the ideas for the further development of phpbb - just look at the stackoverflow ratting system to improve the (B) part of the engine and improve the integration with the static content for part (A).
From technical point of view, the development team is great, and I am really thankful that they
- moved the core engine to symfony framework
- manage to keep the project working with new versions of databases and php 8. Unfortunately many projects died because of changes in php.
- manage to follow the most important trends - like recaptcha or oauth. It is not ideal, but it is working.
I hope for more integration with symfony framework and transition to newer versions of that framework, but of course this a lot of work to be done.
Another (crazy?) idea - maybe at some point a thin version of phpbb could be a module for symphony. Kind of commenting system for static content + general discussion board. And I would remove styling from such a thin version, or make it very very basic to ease its overriding.
But, leaving the wishful thinking aside, phpbb is a very mature piece of software, and I hope it will stay with us in this very difficult and changing environment.