Two-Factor authentication
While I am a pro customer already (yay for daily backups and attachments!!!) Two factor authentication is one thing that I don’t think should be a pro-only. Maybe if you wanted something like SMS for your codes or something, but by way of authenticator apps like Google Authenticator, I would much much rather see this security functionality be available to all.
Two factor Authentication for Account Security is one thing everyone should be able to add to their account, even on a free tier. Security for ones account should be something accessible to all individuals.
That said, I would be okay with secondary methods, like the SMS codes, or YubiKey for 2FA being a paid feature. (I 
my YubiKey for 2 factor) But everything from LastPass, Facebook, Twitter, Google, SnapChat, Tumblr etc all support Authenticator apps for the 2FA at a free level.
That’s just my $0.02CDN on the subject of 2 factor authentication.
Matt