Use remote carddav contacts
I use a same address book on all devices, and can to use it would be useful.
This is a big feature in terms of synchronization.
1) The mobile app does something similar: pulls from the phone contact.
2) For the desktop, once protonmail-bridge is done, you could use it with a 3rd party email client (e.g. thunderbird) which has access to the carddav contacts.
Nevertheless, adding such feature to the service as a whole would just make it more seamless and well integrated. Moreover, protonmail team already pitched a related feedback entry called "Carddav Server For Contacts (Synching Contacts)". This is the other way around, acting as a client.