Currently, I am building a fan based forum addition to my site. Meaning that people are more likely to create creative usernames rather than just having their name shown to the public forum community. Having this feature would be awesome to prevent people from choosing the same username as the official admin/moderators and would protect the creative identity of those who might gain popularity or really commit to their personality within the forum.
I thought about the logistics of this to possibly help the team making all this possible (hopefully it helps)!
Potential Problem #1 - Does everyone want this?
Many sites are different and there might be sites who are perfectly fine with the current forum settings and will have many clients with the same name. So instead of forcing the feature maybe "toggling" this as an option for admins who might want the unique username restriction would be best.
Potential Problem #2 - Username Clash
When members sign up, with the Wix-provided-login-page, they are automatically given a forum username based on their email address... If there is an already existing member who might
have the same username, give the new member a filler username ("newbie245") or just give all new members a unique filler username automatically. (This is also a cool area where the site admin can customize the new member usernames... i.e "newbie245", "fanatic245", "member245", "beauty245")