An option to create an encrypted Backup of all Proton Account data (probably most important are Mail, Pass, Drive and Wallet) which can be downloaded as a File and be decrypted locally with an open-source (FLOSS) software Proton provides and the Proton Account Password (resp. the Decryption Password if two-password-mode is active).

This feature should work exactly the same way as the backup option of Standard Notes does (https://standardnotes.com/help/14/how-do-i-create-and-import-backups-of-my-standard-notes-data):

• Basicly give the user the data the way you (Proton) see it (fully encrypted with the, to you unknown, Account password)
• Make it possible for the user to decrypt the data of ones Proton Account even if Proton goes out of business or the access to all Proton infrastructure is not possible at the moment
• Enable the provision of the encrypted tarball (or file in other format) via E-Mail (as it is encrypted)

The way Standard Notes does it is very very nice (using the existing Account password for decryption and enabling the user to run the decryption tool locally) and also very privacy-preserving and secure (no re-encryption of the data for the backup alone, provide the user with the data in the exact same form the provider has it).

As there is no separate encryption for the Backup itself but just the export of the already encrypted data this can be used for the user to store multiple copies of his Proton data locally, in the cloud etc. and still having one password he needs to remember (of course if he ever changes his Proton Account password it is his responsibility to delete all "old" backup files as they are encrypted using his old Proton Password.