Greetings Nicolas,
Once again, thank you for your suggestion! We're always looking for suggestions and feedback that can help us make Potterworld a better place, and we appreciate you bringing your idea forward!
I’ve brought up your idea up with the team and unfortunately, we are declining this suggestion at this time - this is not something that we will be implementing. We feel that this isn't necessarily needed and it's not really something that we want to add with the upcoming update. It would make tab inconsistent, some players would have usernames and some wouldn't (not everyone is able to purchase a custom nickname). Additionally, it is also important for staff searching for a certain nickname by looking at usernames in tab. Again, thank you for suggestion and we hope that you submit more ideas in the foreseeable future!