The OC could be a wandering ninja who stumbles upon the situation where Kushina is in danger. Maybe they are initially just passing through, but their sense of justice compels them to step in and save her. Then, they might get involved in the larger Naruto world events as a result of this encounter. They could also form a bond with Kushina, which could lead to different relationships with other characters like Minato or Naruto in the future.
A more complex plotline could involve the OC being part of a secret organization. This organization has been monitoring the events in the Naruto world and realizes that Kushina's survival is crucial for the balance of the world. So, they send the OC on a mission to save her. Along the way, the OC has to face many challenges, including other powerful ninjas who are after Kushina. After saving her, the OC may have to deal with the consequences of interfering in the natural order of the Naruto world, such as facing the wrath of certain powerful groups or having to explain their actions to the village leaders.
Yes, there might be. The Naruto fandom is quite large and diverse, so it's very possible that someone has written a fanfiction with such a plot. You can try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3).
A possible plotline could be that Sakura accidentally stumbles upon a hidden memory or dimension where Kushina exists. They then bond over their experiences with the male characters in their lives. For example, Sakura could talk about Sasuke and Kushina about Minato.
One possible plotline could be a what - if scenario. For example, what if Sarutobi had a closer relationship with Kushina before she met Minato. Maybe they could be friends who gradually develop deeper feelings as they face various challenges in the Hidden Leaf Village together.
A possible plotline could be that Tayuya is captured by an evil organization, and Naruto goes on a solo mission to save her. He uses his stealth skills to infiltrate the enemy base.
It could be that a powerful sorcerer in the Avengers' world goes rogue and starts using dark magic. The Avengers don't know how to handle magic, so Harry is called in. He has to face this rogue sorcerer, using his own brand of light magic. Along the way, he might have some clashes with the Avengers' way of doing things, but in the end, they all work together and Harry manages to save the day by defeating the rogue sorcerer and restoring peace in the Avengers' world.
Perhaps Medusa has some crucial information that Percy needs to save the world, and in order to get that information, he has to protect her from other threats. So he embarks on a journey to keep her safe.
One possible plotline could be that Bernardo gets into a dangerous situation, like being trapped in a burning building, and Tony risks his life to save him. Maybe there's a feud between their groups, but Tony's sense of justice or his personal feelings for Bernardo override that.
Perhaps Artemis gets lost in a strange dimension that is similar to Naruto's world. Naruto discovers her there and has to protect her from the dangerous creatures in that dimension. He uses his skills and knowledge of the area to lead her to safety. Along the way, they might form an unlikely friendship or even more, depending on the nature of the fanfiction.
Well, perhaps the OC is in a life - threatening situation like being trapped in a collapsing cave during a battle. Naruto, being the hero he is, uses his shadow clone jutsu to search for and rescue the OC. Or maybe the OC is being mind - controlled by an enemy, and Naruto has to find a way to break the control and save the OC. It could involve a lot of action and some emotional moments as Naruto tries to reach the OC's true self.
One possible plotline could be that Sasuke discovers some hidden knowledge or power that Kushina had, and he tries to learn from it. Another could be that they are brought together in a time - travel scenario, where they have to work together to fix something in the Naruto world's past.