You can plan every "top level" path you'll ever want on the site from now until forever? Or do you mean planning as in plan to force account name changes on users when someone's username conflicts with a new feature name?
Or put all user pages under some top level path and then you never need to ban anything as this problem becomes completely moot.
Until you invent a product or concept name that is taken by one of the users ;-)
In that case it's not really different if the usernames are prefixed to be clearly identifiable as usernames - most companies still wouldn't want any user accounts that look like official company accounts.