This would be awesome, but I don't know that it would be practical given the way that ProtonMail works. If the email service is entirely encrypted and only unencrypted on a user's browser or on an app on their phone, then an email would only be able to send when someone is logged into their account or if they have the app on their phone. I could see it as being possible if someone has the app on their phone that is always running, but I could also see ProtonMail getting a lot of complaints that they scheduled an email that didn't send (which would undoubtedly be a prevalent issue). It also wouldn't be an option available through logging in via a web browser. However, if someone has Thunderbird set up on their PC and they use Proton Bridge, and their computer is running, I'd think that Thunderbird would have some type of send it later feature to make this possible already (I don't know much about Thunderbird though).