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.
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.
It might cause a conflict within the Akatsuki as some members have different views on Sasori's actions. Or it could be the start of a series of strategic battles to defeat Sasori and restore Sakura to normal.
One idea could be that Sasori is transported into a world of puppets where he has to face his own creations turned against him in a strange, new society of puppet - makers. This would put his skills to the test in a unique way.
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.
This transformation would likely have a huge impact. It might change the power dynamics among the characters and create new conflicts and alliances. Also, it could explore the emotional aspects of Sakura's friends dealing with her new state.
The rest of Team 7 and Sakura's friends in the Leaf Village would be shocked and saddened. They would all come together to figure out a way to reverse what has been done. They might seek help from Tsunade, who was Sakura's mentor, and use all the resources of the village to bring Sakura back to her normal self.
In a fanfiction, Deidara and Sasori could be sent on a secret mission to infiltrate a hidden village. Deidara uses his clay creations to distract the guards while Sasori sneaks in with his puppets. As they progress, they discover a powerful weapon that the village is developing. They then have to decide whether to destroy it or take it for the Akatsuki. Along the way, they also have to deal with their own internal conflicts and the pressure of the mission.
They could be on a journey to find a rare material for Deidara's art. Sasori, with his knowledge of the ninja world, helps guide them. Along the way, they encounter various strange creatures and ninja clans. They might have to fight off some who are after the same material.