Web Key Directory support
I propose adding support for modern key lookup - Web Key Directory.
ProtonMail could automatically lookup recipient's key when composing e-mails via WKD and add that key to the contact book.
Some e-mail clients like Enigmail or Mailpile do it that way. There are providers already supporting this like posteo.de and some prominent organizations like kernel.org and gentoo.org
Openpgpjs has support for WKD lookups since version 3.0.12: https://github.com/openpgpjs/openpgpjs/releases/tag/v3.0.12