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.
Bakugou could use his words. He might tell Deku that he believes in him. For example, he could say 'Deku, you may be a nerd but you're a strong nerd.' It's a backhanded compliment but it shows he has faith in Deku.
Well, Deku does lose his quirk. However, this event leads to new developments and challenges for him. It's a crucial plot twist that adds depth to the story.
Bakugou may prove his love by being protective. He might put himself in danger to save Deku. In many stories, this kind of self - sacrifice is a strong indication of love. Also, he could start to understand Deku's dreams and support him in achieving them, which is a deeper form of love than just friendship.
Well, Deku's arm loss happened when he faced an adversary whose abilities were beyond what he could handle. It was a high-stakes moment that had a huge impact on the story.