Deku could be a main character. Since his whole journey has been centered around his quirk, losing it would be a huge deal. He would have to face the fact that all his training and growth related to his quirk was now useless, and he'd have to start from scratch in a different way.
It could be Bakugo. He's always been so proud of his powerful quirk. Losing it would be a major blow to his ego. The fanfic could focus on his struggle to come to terms with being 'ordinary' and how he might try to regain his status in a world without quirks, maybe even using his intelligence and combat skills he had before relying solely on his quirk.
All Might could also be a main character in such a fanfic. He's already in a weakened state, and losing his quirk completely would be the ultimate test. He could serve as a mentor to the younger heroes who are also dealing with the loss, sharing his experiences of being a hero even without the all - powerful quirk he once had.
A 'bnha losing quirks fanfic' might explore a situation where a mysterious force from another dimension is sucking out the quirks. This causes chaos as the heroes struggle to protect the civilians who now see them as just normal people. The story could then be about the heroes' journey to find a way to reverse this, learning about the true essence of heroism that goes beyond just having powers along the way.
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.
All Might could also be a main character in a bnha alien fanfic. His status as a symbol of peace in the BNHA world could be extended to an intergalactic scale. He could be the one to bridge the gap between humans and aliens, using his experience and power to negotiate and fight when necessary. His weakened state could even be a plot point, as he has to rely on new alien technology or powers to regain his strength and continue to protect.
Dabi would also work well. His vendetta against Endeavor could be a starting point for his reform. As he discovers more about his family and the truth behind his father's actions, he might decide to change his ways and use his power for good instead of revenge.
Maybe Deku could be the main character but with a temporary one - punch quirk. He could gain this quirk through some sort of experiment gone wrong. And he has to learn how to use it while still relying on his original 'One For All' quirk in certain situations. This would add a new layer of complexity to his character development.
Maybe in a magical academy setting, where students are encouraged to develop and use their quirks for various tasks and challenges. Such a place would provide the freedom and opportunities for everyone to showcase their unique abilities.
Mustard is, of course, one of the main characters. Depending on the fanfic, other characters from the BnHA series like Deku or Bakugo might also be involved. They could be either in a confrontational relationship with Mustard or in a situation where they are trying to understand him better.
It could be a young and inexperienced trainer who has big dreams but is still learning the ropes. Losing the battle would be a huge setback for them.
Often, the main character would be the one with the stretch quirk. They are usually the focus as they navigate the world of BNHA with their unique ability. There might also be a mentor figure, someone who has more experience with quirks and helps the stretch - quirk character develop their skills. And of course, there could be friends or allies who team up with the stretch - quirk character in battles or adventures.
Obviously, Opbozuku is one of the main characters. Often, his friends and classmates from the BNHA world also play important roles, like Deku, Bakugo, etc. They might be his allies, rivals, or both in the fanfic.