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 plotline might involve an alternate universe where the political and social structures of the village are different. This could lead to Kushina and Sarutobi being paired together for a mission that requires them to pretend to be in a relationship, but then real feelings start to develop over time. It could explore how they navigate their new - found emotions while still being part of the ninja world.
Perhaps a plotline could center around a time - travel concept. Kushina or Sarutobi could be sent back or forward in time and end up in a situation where they have to rely on each other. This could create a unique dynamic between them as they try to figure out how to get back to their own time while also dealing with their growing attraction. This type of plot could also involve them using their knowledge of the future or past to change certain events in the village's history.
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.
A more complex plot could involve time - travel or alternate universes. For example, an alternate universe where the universes of Dragon Ball and Naruto are merged. Goku and Kushina meet under these strange circumstances and have to figure out how to deal with the chaos that ensues while also exploring their new - found feelings for each other. This could involve battles against powerful villains who want to take advantage of the merged universes, and Goku and Kushina would be at the forefront of protecting their new world together.
One plotline could be that Naruto discovers a hidden power within himself that is linked to Kushina's Uzumaki heritage. This power makes him godlike, and he has to learn to control it while also dealing with the memories and emotions related to Kushina that come with it.
One possible plotline could be that Naruto is influenced by some dark force and turns 'evil'. Kushina, having come back to life somehow, tries to understand what has happened to her son. She then embarks on a journey to find a way to reverse the effects, leading to a series of adventures where they interact in different ways, like Kushina trying to jog Naruto's memories of their family bond.
I'm not sure about specific Kushina x Sarutobi fanfictions off the top of my head. You could try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. There might be some interesting stories there that explore their relationship in different ways.
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 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 common theme is friendship. Given their different positions in the village, a story could be built around how they become friends. Another theme could be protection. Since Kushina is a jinchuriki, Hiruzen might be involved in protecting her from those who would seek to use her power for evil.
One possible plotline could be Naruto's struggle with abandonment issues. Since Kushina left, Naruto might have a harder time finding his place in the village. He could become more withdrawn and have to fight even harder to prove himself. Another could be the search for Kushina. Naruto might embark on a journey to find out why she left and try to bring her back.
Yes, there are quite a few. Some fanfictions might focus on their interactions within the Hidden Leaf Village. For example, they could be involved in a joint mission where Hiruzen, as the experienced Hokage, imparts some of his wisdom to Kushina. This could lead to a great story of mentorship and friendship.