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.
A really interesting Vampire Deku fanfiction is 'The Vampire Deku Chronicles'. It takes place in an alternate universe where vampires are a common part of society. Deku becomes a vampire and uses his new abilities in unique ways to fight against the villains that threaten both the human and vampire worlds. It has a very engaging plot with a lot of twists and turns.
Another theme is the relationship changes. His friends might react differently to him as a vampire. Some might be scared, while others might try to help him. This can lead to a lot of interesting character dynamics in the fanfiction.
You can try fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. They usually have a large collection of fanfictions, including ones related to 'My Hero Academia' with a Vampire Deku concept.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
His friends could come to the rescue. Bakugo, for instance, might use his explosive power to break into Toga's hideout. He's always been competitive with Deku, but he also cares about him deeply.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
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.