In this fic, MC will be reborn as Sasori of the Red Sand's older brother. MC will be three-four years older than Sasori, and graduates the academy at seven, becoming chunin at eight, just like the original Sasori.
MC can either be a puppet master and poison maker like future Sasori, or medical-nin like Chiyo.
Write about the relationship between Sasori and MC, as well as one with Chiyo and their parents before their death.
MC participates in the Second Shinobi War, and witnesses the deaths of their parents. Sakumo spares him with severe injuries out of kindness after seeing his parents die trying to save him.
Write about MC's bond with Sasori, and about how he comforted Sasori with his father-mother puppets (e.g. helping him with his puppetry skills and lifting them up in the iconic falling scene.)
In the Third Shinobi War, MC dies in battle, starting Sasori's descent into darkness.
Chiyo makes him a puppet based on MC, like the father-mother puppets to help him cope with the loss.
Cannon happens, Sasori turns himself into a puppet and defects.
After Sasori joins the Akatsuki, and the Kazekage Rescue Arc starts.
Chiyo and Sakura battle with him like in cannon, but this time, he was killed with both the mother-father puppets and also the 'brother' puppet.
In the Fourth Great Shinobi War, both Sasori, Chiyo, and MC get Edo-Tensei'd.
MC meets with Sasori and after a reunion, he convinces Sasori to let Kankuro and the others seal them away.
Both entrust their old puppets, as well as the mother-father puppets to Kankuro, and die peacefully.
The rest of the story or the gaps in between are up to you, and you can change the story however you want, just make sure the concept is still the same.