One key element is the resolution of their past conflicts. Since they had a lot of rivalry in the original story, in a friends - based fanfic, they need to address those issues. For example, Bakugou realizing his wrong attitude towards Deku.
One antagonist could be Orochimaru from Naruto's world. He could be interested in the power of Quirks in Deku's world and try to capture Deku to study his power. He would be a formidable foe as he is a master of ninjutsu and has a lot of devious plans.
One possible plot could be Naruto and Deku finding themselves in a strange new world together. They have to combine their unique skills, Naruto's ninjutsu and Deku's One For All power, to face the challenges of this world and find a way back home.
One great fanfic is 'The Road to Friendship'. It shows Bakugou slowly realizing Deku's worth and them building a bond through various challenges at UA. It has some really heartwarming moments like when they team up in a training exercise and start to understand each other better.
Well, Iida would probably have a very proper reaction. He might give Bakugou a stern talk about treating Deku right and being respectful on the date. Mina would be super excited and would start gossiping about it right away. She'd be all over Deku, asking him what he was going to wear and things like that. And Kaminari would probably make some silly jokes about it, like saying Bakugou was finally 'going soft' for Deku.
One plot idea could be that Bakugou and Deku are paired for a really difficult hero training mission. At first, they bicker as usual but then realize they need to work together. They start combining their quirks in unique ways and in the process, they become closer as friends. Another idea is that they have to protect a small village together from a powerful villain. During this, Bakugou learns more about Deku's selfless nature and starts to respect him even more, deepening their friendship.
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 this fanfiction, Deku's character development could be quite complex. Going back in time, he may initially be overconfident because he thinks he knows what's going to happen. But as he realizes that changing the past has unforeseen consequences, he becomes more cautious. He also might develop a deeper understanding of his friends and their potential, as he gets to see them at an earlier stage. He could start mentoring some of them, which would in turn make him more responsible and mature as a character.
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 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.