I can see two problems. One is where do you draw the line - e.g. what if someone said "I want my organs to be given to a white person.".

The other is with allowing people to specify friends as recipients - if someone is in need of a donor it would be tempting for them to seek out someone with a terminal illness & persuade them (e.g. by payment of money) to claim that they were personal friends & wanted to donate their organs to that person.

I can see how it might be possible to make an exception for blood relatives, but I think it would need to be carefully worded & controlled.