Seeing how long this community and site has existed for, this is most likely not the first time someone has suggested national anthems for user's nationstates. I think it would be a great idea as it will contribute to each nation's identity and give more insight to others what the nation is all about, because a lot of information and emotions can be found and interpreted through music. Nations could unlock this feature by exceeding a certain population number (just like capital city, national religion etc.). Perhaps, the user could enter a YouTube link or upload a music file for their selected anthem, and it can await approval or be ready for moderation. Once approved, their song can be displayed on the side of their nation's page under the box with their population, capital city etc.
Thank you.