Usernames are a much, much harder problem than what you might think at first glance… even if you can get away with a really simple and naive implementation on a prototype, a large, global and secure service must consider lots of not-so-obvious details and possible attack vectors.
Let’s talk about usernames deals with the problem with uniqueness, homograph attacks, confusables and other security concerns that you might need to consider.
It is now well known that people use technology in unexpected ways (at least, in ways that software engineering and product teams had not intended) […] Our original charge was to find ways to improve and optimize users’ browser workflows following software and design-oriented assumptions. Instead, we saw that users were doing just fine with the tools they were already using.
In Praise of Theory in Design Research: How Levi-Strauss Redefined Workflow