If Tomura saved Deku, it might start a process of redemption for Tomura. Deku could try to understand him better and perhaps find a way to bring him to the side of good.
It could completely change the dynamics of the story. Maybe Deku would have a different perspective on villains and their motives.
It could lead to a major shift in the story's direction. Deku might have a different perspective on heroes and villains.
Dabi could be a great character to pair with Tomura. They are both villains and their powers and personalities could create some interesting interactions. Maybe they could team up for a particularly difficult mission or have a power struggle within the League of Villains.
One idea could be exploring Tomura's backstory in more detail. Maybe show his life before he became a villain, his family situation, and what led him to the path of destruction. Another could be a 'what if' scenario, like what if Tomura was redeemed and joined the heroes. He could use his knowledge of the villains to help the heroes take down other threats.
Overall, Tomura's mask in the My Hero Academia manga is designed to make him look more menacing and intimidating. It often plays a role in emphasizing his evil presence and sets him apart from other characters. The mask's details and style contribute to the overall atmosphere of the story.
One idea could be exploring Tomura's past in more detail. Maybe show his childhood before he became a villain, how he was influenced by All For One. Another could be a story where Tomura tries to redeem himself, which would be a huge twist from his normal villain role.
Shigaraki Tomura is one of the main antagonists. He's a powerful villain with a complex backstory and motives.
Give Tomura a new power or ability that is unique to your fanfiction. This will set your story apart from others. For example, he could have the ability to control time for a short period.
Toga could be an ally. Since she has her own complex relationship with the idea of heroes and villains, she might find Deku's transformation interesting. She could provide some of her unique skills, like shape - shifting, to help Deku in his villainous plans. Additionally, Twice might also join them because of his loyalty to Toga and his own sense of being an outcast in the hero world.
In fanfiction, a quirkless Deku often has to rely more on his wits. Canon Deku got One For All, but in fanfic, he has to find other ways to be strong. For example, he might study different fighting styles more intensively.
It could lead to a major shift in the story's direction. Deku might have a different perspective on villains and heroism.