Allow developers to implement their own apps integrated to the Proton enviroment and, through an auth flow, get the required permissions to use users accounts

Examples of implementation
Proton Drive SDK (Existing Proton Drive SDK implementation of authentication and app indentification is easily forjable and apps wth bad intentions will store user credentials)
A future Proton Mail SDK (which would allow open email clients like Thunderbird to implement Proton Mail without the need of Proton Mail Bridge)
Proton VPN SDK (An SDK where apps can tunnel requests via a Proton VPN server)
Proton Docs & Calendar SDK (SDK to develop Proton Docs addons [available via the Proton Docs website] and apps that read Proton Docs data)

Some implementation suggestions
The user should be warned about every permission the app is asking
The authorization URL should be very clear that gives access to your Proton account (such as "authorize.proton.me)"
A form of 2FA should be asked before authorizing any app

This is an important step for a free environment, where Proton apps and third party apps have a secure auth flow with proper and granular permission granting