Start with clear categories. System terms are always blocked — no exceptions. Brand names are blocked or require verification. Celebrity names might be restricted pending identity verification. Premium handles could be reserved for paid accounts. Each category needs explicit rules.
Define your appeals process. When a user believes their username was incorrectly blocked, what happens? Who reviews the appeal? What documentation is required? Platforms like Twitter and Instagram have formal dispute processes for trademark claims.
Document your enforcement approach. Are blocks automated or manual? Do restricted usernames go into a queue for human review? What happens to accounts that somehow registered restricted usernames before blocks were in place?
Plan for edge cases. What about parody accounts? Fan accounts? Legitimate name collisions (someone actually named "John Google")? Your rules need to address these scenarios before they become support tickets.