Allow a user to publish a shared folder in their mail db and create a link to it which other users could add to their own mail db's left-hand navigator/outline in the mail db. Perhaps "Other Folders"
This is not entirely unlike the "other mail" or "show calendars" calendar federation options currently offered, where Notes essentially links to the calendar view of the other user. This is most useful for mail-in databases where it would be useful for a team of people to see the contents of one particular folder from the mail-in db.
The link created should open within the context of the current user's mail view frame, perhaps with a special heading to indicate it is a shared folder from another db. It should NOT open into another tab. Users should also be able to easily drag/copy docs from the shared folder to their own mail folders.
A recent customer experience prompted this idea, as the broader idea of shared folders from another product was widely used in the organization.