ProtonMail security features with other email services.
Allowing customers to use ProtonMail features with a gmail account with advertising auto message, "This gmail message was processed using ProtonMail high level security" attached. That service would be huge. I'd even pay for such a service where I could use my gmail account to send a ProtonMail secured message. It's a pain to change over primary email provider to ProtonMail. Everyone wants security, but the difficulties of notifying all their contacts about a new email address is daunting.