It would be great if 2 factor authentication (in any form) could be added to mattermost (or even the whole gitlab suite).
For me that would mean that I can explore mattermost as a website to the internet, allowing colleagues to use it from anywhere including mobile devices.
Currently we use Yubikey's, so a simple radius authentication hook would be sufficient, as we can add the otp token to the password (a bit difficult on mobile, but not impossible).
But any form of 2 factor would do for us.