Subscribe user-selected IMAP folders via PM Bridge
Any full-featured IMAP client allows a user to "subscribe" to (and to search for the names of) any folder in an IMAP server. As far as I can tell, this feature is not available to anyone using Bridge v1.2.3, no matter what email client they are using. My client is Outlook Pro 2016.
If implemented, this feature will allow Bridge users to use their existing email client to archive the messages in their PM IMAP folders -- allowing us to fold these messages into our already-existing email-archiving systems. (At present, we are able to use https://mail.protonmail.com/ to file email messages in a webfolder that's labelled "Archive", but there's no way to bulk-download them AFIK.) Minimising the number of messages in the subscribed folders of an IMAP connection is very important for Outlook users, to avoid the performance problems caused by its (stupidly) downloading entire messages (with attachments) during every sync, rather than downloading only headers in the first phase of a sync operation (as any performance-tuned IMAP client will do).
If implemented, this feature will also make it possible for Bridge users to develop their own sorting system (with automated filters) so that their incoming email is conveniently sorted into folders that are visible on their email client, rather than labour-intensively inspecting everything in an Inbox.
If implemented, then PM's spam-filtering would become helpful for Bridge users, rather than a trap for the unwary (who haven't yet noticed that they should disable PM's junk-sorting filter, if they don't want to -- or can't reliably remember to -- login to https://mail.protonmail.com/ periodically to look for inaccurately-sorted Junk.