The private replies might be abled to be forked perhaps to come up with something along your idea. But will require someone whom knows what there doing or discussing maybe with the plugin author if maybe a sponsored plugin fork. With sponsoring this would be great to explore a form of crowd funding concept.
The group mailbox is a great idea, however imho the PM system needs to have an easy to use Search option, or Group BookMarks with using a Folder option. Ie Client A Oct 2020 requests.