One possible plot development is that they team up to take on a common enemy that neither of their villages could handle alone. This enemy could be a powerful rogue ninja or a group with a dangerous new jutsu.
In the fanfiction, Naruto joining Gaara could result in a quest to find a lost or hidden power. They might discover ancient scrolls or artifacts that are related to their Kekkei Genkai or some long - forgotten ninja art. Along the way, they would have to overcome various obstacles and challenges, which would strengthen their bond and make them even more powerful as individuals and as a team.
There could be a plot where they discover a new threat to the villages while living together. They then use their combined skills and knowledge to investigate. Gaara's knowledge of the desert and its secrets could be combined with Naruto's intuition and quick - thinking. They might also find some ancient power or technique during their investigation that could change the balance of power in the ninja world. This would lead to them having to decide how to handle this new power, whether to keep it secret or share it with the other villages for the greater good.
A plot development could be that Gaara and Sasuke start the fight. Sasuke tries to taunt Gaara about his past and how he could never truly understand Sakura like he does. Gaara, in response, shows his determination through his sand attacks. As the fight progresses, Sakura starts to see a side of Gaara she never knew, which makes her question her feelings for Sasuke. Meanwhile, Sasuke becomes more and more frustrated as he realizes he might be losing Sakura's attention, which makes him fight even more recklessly.
Perhaps Percy joins a special unit in the military. In this unit, he discovers some sort of conspiracy or mystery within the military structure. This could lead him on a journey to uncover the truth while still fulfilling his military duties. He might also face some moral dilemmas along the way as he tries to do what's right for the military as a whole.
Well, it could be that her joining Voldemort causes a huge shift in the balance of power. The wizarding community would be in chaos as they try to figure out how to deal with this new threat. Dark Fem Harry might also start to influence Voldemort in unexpected ways. Maybe she convinces him to use more psychological warfare rather than just brute force. And she could be the one to find and destroy some of the remaining Horcruxes of other wizards, believing that it would make Voldemort even more powerful and thus her position more secure.
One possibility is that Naruto's friends launch a rescue mission. They would use their unique skills, like Sasuke's Sharingan or Sakura's medical ninjutsu combined with her combat skills, to find and save Naruto.
For Naruto, he could develop a deeper understanding of the pain that Gaara has been through. This might make him more empathetic towards others who seem hostile at first. He might also learn new combat techniques from observing Gaara's use of sand in the heat of the Chunin Exams battles.
Well, they could form an unbeatable team. Naruto's positive energy and Gaara's cool-headedness would complement each other. They might go on adventures to protect their villages together, sharing their unique jutsu and strategies along the way.
Gaara might start to show more of his soft side. With Sakura by his side, he could become less isolated and more involved in the social aspects of his village. His leadership style could also change, becoming more collaborative. Sakura, she would become more in tune with the desert environment and the needs of the Sand Village. She would probably develop her fighting style to incorporate some of the Sand techniques, which she learns from Gaara. Also, her understanding of love and family would deepen as she builds a life with Gaara.
For Sakura, she might become more confident in her combat abilities. Working with Gaara, who has such strong and intimidating power, could push her to step up her game. She could also develop more empathy as she gets to know Gaara's troubled past. For Gaara, he could start to open up more to others. Sakura's kindness and determination might make him realize that not everyone is out to harm him.
In a fanfiction where Naruto joins the Akatsuki, there could be a lot of interesting character interactions. He would be in close quarters with villains like Sasori or Hidan. Naruto might try to learn more about their pasts and what led them to become members of the Akatsuki. This could also cause a rift in his relationships with his old friends in Konoha. They might see him as a traitor, while Naruto is actually trying to bring about change in the Akatsuki for the greater good.