They could start training together more often. Since Tsunade is a great medical ninja and has a lot of combat experience, she could teach Sakura some advanced medical and combat techniques. This shared learning experience would strengthen their bond.
For Naruto, he could develop by learning more about leadership from Tsunade. He already has his own unique style, but Tsunade's wisdom in leading the village could give him new insights. Sakura could develop her medical ninjutsu further under Tsunade's guidance and also become more confident in her combat abilities. Tsunade herself could develop by seeing the new generation, Naruto and Sakura, grow and be inspired to take on new challenges for the village.
Their relationship can be developed through shared experiences in the medical field. For example, they could work together on a difficult medical case that requires both of their skills. This would force them to communicate, trust each other, and learn from one another.
Their relationship could be developed further through mutual understanding. For example, Naruto could learn more about Kurama's past and the reasons for his anger and hostility in the beginning. This would make Naruto more empathetic towards Kurama, and Kurama in turn could start to trust Naruto even more.
In fanfiction, one way to develop their relationships is through a series of challenges. Let's say they are all put in a survival situation. Neji's calm and calculating nature would help them plan. Sakura's medical skills would keep them alive. Naruto's determination would keep their spirits up. During this time, they would learn more about each other's fears and hopes. Naruto might realize that Neji has a soft side, and Neji would see Naruto's growth. Sakura would become closer to both of them as they rely on each other. This would strengthen their relationships in different ways, like a bond forged in the fires of adversity.
For Naruto, you could show his growth in understanding his feelings for Sakura. Maybe he starts to realize that his protectiveness towards her is more than just friendship. He could start to show small gestures of affection that are more romantic in nature.
For Naruto and Sarada, Naruto could become a more active mentor. He could take Sarada on special missions and during those, they could bond over their shared love for the village. Naruto could also share his experiences as an orphan, which would make Sarada feel closer to him.
One way is through joint training sessions. Sarutobi Sensei could push Naruto to his limits and in the process, they would learn more about each other's strengths and weaknesses. Also, they could be put in a situation where they have to rely on each other completely, like being trapped in a dangerous genjutsu. Sarutobi Sensei could then use his wisdom to guide Naruto out, strengthening their relationship. Another aspect could be Sarutobi Sensei introducing Naruto to important figures from his past, which would give Naruto more context about his sensei and bring them closer.
It can start with mutual respect. Naruto respects Tsunade as the Hokage and for her great strength. Tsunade, in turn, can see Naruto's potential. For example, Tsunade could start mentoring Naruto more closely, like sharing her experiences during the wars which Naruto hasn't been through yet. This would create a bond between them.
Another way to develop their relationship is through a common goal. Let's say there is a new threat to the village that requires both their skills. Tsunade's medical expertise and Naruto's combat abilities are both needed. As they work together towards this goal, they learn to trust each other more, share their techniques, and their relationship evolves from acquaintances to comrades and then to close friends.
Their relationship can be developed by showing them facing challenges together. For instance, when Naruto has to make tough decisions as Hokage, Hinata can be there to support him morally. And when Hinata has to deal with her family's expectations, Naruto can encourage her to follow her own path. This mutual support can deepen their connection.