Sasori is definitely a main character in this 'sasori love story'. As for other possible main characters, it could be someone who is close to Sasori, like a friend who helps him in his love journey, or the person he falls in love with. This person could be from the same world as Sasori or a complete stranger that he meets by chance, and together they form the core of the love story.
Obviously, Sakura is one of the main characters. She is a strong and determined kunoichi. And then there's Sasori's sister. We don't know much about her yet, but she's likely to be a complex character, perhaps influenced by Sasori's actions and reputation in the ninja world.
One possible motive could be Sasori's desire to create the perfect puppet. Sakura, being a strong kunoichi, might have physical and chakra attributes that Sasori finds appealing for his puppet - making. He may see her as a unique specimen to add to his collection of puppets.
The Akatsuki could still be the main antagonists. Even though Sasori is training Naruto, the rest of the Akatsuki might view Naruto as a threat and try to eliminate him. Also, Orochimaru could be an antagonist. He might be interested in the new skills Naruto is learning from Sasori and try to steal them for his own experiments.
Well, clearly Sasori and Sakura are the main characters. Sasori is often depicted as having a rather mysterious and brooding personality. He might be into some dark arts or be an outsider in the school society. Sakura, on the other hand, could be a popular girl, perhaps involved in school clubs like the cheerleading squad or the student council. She is usually portrayed as being kind - hearted but also having a strong will.
Obviously Naruto and Sasori. Naruto as the'son' would be the central figure in terms of the story's development. And Sasori, as his 'father' in this fanfiction, would play a crucial role in shaping Naruto's growth.
It could still be Orochimaru. Even though the focus is on Naruto and Sasori talking, Orochimaru might be pulling strings in the background. He could be using Sasori's connection to Naruto to further his own evil plans, like getting access to some powerful jutsu that Naruto might possess or know about.
When writing Sasori's character interactions in a crossover, consider his past. He has a lot of trauma related to his own transformation into a puppet. If he meets a character who has also faced some form of self - alteration or loss of humanity, there could be a deep connection. Maybe they start by sharing their stories hesitantly and then form an unlikely bond based on their shared experiences.
In the fanfiction, they could have a mentor - mentee relationship. Naruto, with his big heart and positive attitude, could be the one to show Sasori a different way of looking at things, like a mentor. Or Sasori could teach Naruto some of his puppet - related skills, acting as a mentor in that aspect.