Multifactor Authentication e.g. Authy, Google Authenticator
I know this could be a plugin, but i think having it integrated and have the option to enable it would be sweet.
Admins could decide whether only certain groups or everyone has access to use it.
I think this would also increase the security of NodeBB as a whole :D
Nice to have.
On my to-do list. But it's down there a ways.