Perhaps Sasuke, being the proud Uchiha, tries to hide his concern at first. But the other villagers notice and start to nudge him towards Naruto. When he finally goes to see Naruto, he finds that Naruto is still as upbeat as ever despite being sick. This makes Sasuke respect Naruto even more, and he decides to stay with Naruto throughout his illness, and they end up having a stronger bond after Naruto recovers.
One idea could be that Sasuke, despite his usually stoic nature, feels a sense of worry when he hears Naruto is sick on his birthday. He could visit Naruto and bring him some traditional Uchiha herbal medicine that he secretly prepared. This would show a soft side of Sasuke and also deepen their bond in the fanfic.
The fanfic could explore how Sasuke deals with his emotions when Naruto is sick. He might be conflicted between his old rivalry feelings and his newfound, deeper feelings for Naruto. Sasuke could also be the one to stay by Naruto's side when everyone else is busy. He'd watch over Naruto while he sleeps, and when Naruto wakes up, there could be a moment of intense eye - contact that shows the unspoken bond between them.
In most 'Naruto gets sick' fanfics, his friends are very concerned. For example, Sakura will immediately start using her medical skills to diagnose and treat him. Sasuke, although he may not show it outwardly as much, will stay by Naruto's side and keep a watchful eye. Shikamaru might come up with plans to make Naruto more comfortable during his illness. And Hinata, she would be really worried and would probably bring Naruto some home - cooked food to help him regain his strength.
Sasuke could play a crucial role. He might use his Sharingan to detect what's wrong with Naruto more accurately. Then he could go on a mission to find a special cure.
His character would likely become more complex. With the Sharinegan, he'd have to deal with new responsibilities and a different kind of power. He might become more introspective as he tries to understand the origin and purpose of this new ability.
The plot could start with the other characters being oblivious to her yandere nature. As the story progresses, her actions would start to raise suspicions. For example, if there's a training session and she gets overly aggressive towards someone who accidentally hurt her love interest during sparring.
Well, it could start with Sasuke joining the Akatsuki with the sole intention of protecting Naruto from within. He could form alliances with some of the less - evil members who might also have a grudge against those who want to harm Naruto. For example, he could team up with Zetsu, who has his own motives and might be persuaded to help protect Naruto in exchange for something.
One way it could develop is that the baby has some special powers that attract the attention of villains. Naruto and Sasuke then have to protect the baby while also trying to teach it right from wrong. As the baby grows, it could start to have its own dreams and goals, which might be different from what Naruto and Sasuke expect, adding more drama to the story.
One possible plot could be that Sasuke has been away on a long mission and has forgotten his own birthday. Naruto, who has been keeping track, decides to bring the celebration to him. He travels to where Sasuke is and with the help of some local people, arranges a small, intimate party. They share stories of their adventures and Naruto gives Sasuke a new headband as a gift, one that represents their combined journey.
A 'Happy Birthday Sasuke' Naruto fanfic could be about Naruto planning a surprise birthday party for Sasuke. He might gather all their friends from the village, like Sakura, Kakashi, and Shikamaru. They could decorate Sasuke's old house with symbols of his clan, the Uchiha crest perhaps. Naruto would also make sure to have Sasuke's favorite food, tomatoes, available in abundance. It could be a story full of friendship and nostalgia, showing how much Naruto values Sasuke.