Enforcing 2FA on anyone inside your organization.
Setting company-wide signatures.
Using HTML as your signature.
Setting company-wide/user details (ie. Allowing appending "Regards, %%Position%% %%LandLine%%" on an outgoing email).
Mail-flow rules (though admittedly, this is basically the same as the previous point).
Tagging of external emails ("This email was received outside of the organization at the start/end/subject of an email).
These are a few QOL suggestions i can think of off the top of my head. However, the enforcing 2FA on users, and being able to tag external emails are outright security issues which should genuinely be implemented as ProtonMail is both security and privacy focused. I did raise a ticket with all of this in a few months ago, but hope this gives the devs some visibility so they can use that sweet EU funding to improve on these. These basic features would make a world of difference to all users.
The use of HTML as your signature (last time I checked anyway) is silly, as this actually works absolutely fine. We ended up manually pasting the HTML into the page using right click -> inspect element, and hitting save... this works fine if anyone else is having the same problem.
TL;DR: Highly recommend ProtonMail but only for small orgs at this point.