Manual "Expire" (Self-Destruct) Button
Manual "Expire" (Self-Destruct) Button allows the user to manually expire an email at any time or expire a self-destruct link before the expiration time.
As it is, let's say your recipient received your private link and read the message one hour after you sent it, but the self-destruct time was set for 48 hours. The email link will still be accessible until the expiration time. A manual Expire button would resolve that.
This idea above may be more difficult to do than it seems, but a similarly useful added function could instead be an encrypted e-mail sent to a non-PM user that will expire after being opened / decrypted a certain number of TIMES. This would make possible e-mails that would expire after one or several decryptions (i.e. viewings), rather than expiry by time limit. In this case, Proton Mail could implement expiry by TIME or by VIEWINGS and when setting the expiry method, the PM user could select by hours or by views for message expiry, making the expiry function more versatile depending on the nature of the particular e-mail in question.