I don't think you understand how severely that would destroy the privacy offered by Protonmail.

I do understand that you want a common account to log in to various sites, and that is a valid request considering the market dominance of Google and Facebook. But Protonmail is NOT the appropriate service to provide common logins. I know that there are some authentication providers out there, but I have not looked recently. I heard about OpenID, but I also vaguely recall that the service was discontinued or became unreliable.

Anyway, please try to learn more about on-line privacy and the different ways that you can jeopardize it without realizing.

`protonmail account` to log in on any other website.

When you try to log into some websites they give the opportunity to sign up or to log in with an existing google account or fb account.
Why not trying to offer the same with a protonmail account? that would make me more confident my privacy is more respected.
I would like to see the possibility to use or not, this feature in protonmail.

I am also building a platform and plan to have quite a lot of users, or certainly reach out 300k potential users with mkting campain.
I would love to allow my users to log in with a protonmail account or even force them to create a protonmail account in order to use my website.

That would help protonmail to grow its users and users would have their privacy respected.

I think it would be great, if ProtonMail would provide an OAuth 2.0 service.
This would be my main login for all my profiles on the web.