client AND server would be most beneficial, allowing contacts to be either managed within protonmail or by a third service (owncloud/nextcloud, icloud, etc)

This can be done already. For instance, if you have gmail, you can go to https://contacts.google.com/
From there you can export an outlook contacts file. Then, ProtonMail has an option to import such a file. This is better than connecting to your account because it is more anonymous this way.

Please add the ability to sync with mobile contacts on mobile devices. This way we can use ProtonMail as our primary place to store and sync contacts with phone numbers as well as email address.

Yes, please add this feature! I would like to move all my stuff from Gmail to ProtonMail!

Please add this :-)

Having access to cardDAV (and calDAV) are my practical show-stoppers from switching to ProtonMail. I use google to manage my contacts, and then use cardDAV on all my devices. I live in two countries, and have two phones, multiple other devices, and I need some way of syncing all my contacts....

Would be perfect for Owncloud or Synology NAS completion.

Good sugestion, will be a very good enhancement.

Please add support for CardDAV and CalDAV (add calendar feature)

Then would be much easier for people to migrate from gmail...

Want to sync my contacts \o/ (owncloud)

Contact synchronization for all apple device.

It would be nice if protonmail could both act as a server and a client. That way, when you already have a carddav server running elsewhere, you can sync protonmail contacts based on that.

Carddav Server For Contacts (Synching Contacts)