Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.
It could lead to a completely different character dynamic and story arc. Maybe she'd face different challenges and have a unique perspective on being a hero.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
Another plot might involve the older Deku being from an alternate timeline. He shows the younger Deku different ways he could have developed his skills and relationships, which makes the younger Deku reevaluate his own path as a hero.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
One great Deku and Ochako fanfiction is 'The Path to Heroes'. It beautifully portrays their relationship development. Another good one is 'Deku and Ochako: Training Days' which focuses on their training together and how it deepens their bond.
If Deku had a quirk in fanfiction, it would likely impact his character development. He might have become more powerful quicker or taken on different enemies. The story could become much more action-packed or focus on different themes.
Naruto's outgoing and boisterous personality would make Deku more confident. He would probably be more upfront and less shy. Naruto's strong sense of justice would also intensify Deku's already existing desire to help others.
The relationship could develop slowly. Deku would likely admire Tsuyu from afar at the beginning. Then, they could be involved in group activities where they start to work together more closely. Deku would notice Tsuyu's kindness towards others and her determination. Tsuyu, in turn, would see Deku's growth and his unwavering spirit. As they get to know each other better, their relationship would progress from acquaintance to friendship and then to something more romantic.
One common theme is the struggle with identity. When Deku becomes a vampire, he has to figure out who he is now. Is he still the same hero - in - training? Another theme is the power struggle. Vampire powers can be both a blessing and a curse, and he has to learn how to control them without losing himself. For example, in some fanfictions, his new vampiric strength is difficult to manage at first.