Midoriya could be a great main character. His growth throughout the series makes him a natural choice. His graduation could be about finally proving to himself and others that he has become a great hero, ready to face the world on his own.
Bakugo could also be the main character. His fiery personality and his own journey of self - improvement would make for an interesting 'bnha graduation fanfic'. His graduation might be about learning to work better with others, a big step for his character development.