Seamlessly send encrypted mails to other encrypted mail providers
I honestly don't know how much work it would be, but wouldn't Tutanota's encryption function be more useful if encrypted mails (without clicking a link + typing a password) could be sent to users of other encrypted mail services (protonmail, hushmail, etc.) instead of just Tutanota users? Perhaps this would mean Tutanota, Protonmail, Hushmail, etc. would have to trust each other a bit more, or work out some additional standards for the encryption, but uniting with others who have similar ideas to achieve your (plural) goals can make all involved stronger, and plain email works so well simply because the various providers don't differentiate between external and internal addresses/users when sending messages or displaying received ones.