One idea could be a story where Itachi and Kushina meet in a parallel universe. In this universe, the events of the original Naruto storyline are altered, and they end up on the same side, fighting against a new common enemy. Their unique abilities, Itachi's Sharingan and Kushina's powerful chakra, would make for an interesting combination in battles.
Another fanfiction concept could be a time - travel story. Kushina travels forward in time to a point where Itachi is still alive. She initially mistakes him for an enemy but then realizes his true nature. They could form an alliance to prevent a future disaster that Kushina has knowledge of from her own time. This would involve a lot of character development as they learn to trust each other.
One idea could be a time - travel fanfiction. Naruto Uchiha (assuming it's an AU where Naruto has Uchiha heritage) somehow travels back in time and meets Kushina Uzumaki. They could form an unexpected friendship as he tries to protect her from the events that would lead to her death. He could also use his knowledge of the future to change things in the village for the better.
A really unexpected plot twist could be that Kushina has some secret power that she's been hiding, and it's the only thing that can save Itachi from a certain death. But in order to use this power, she has to sacrifice something very dear to her. This would create a tense and emotional situation where both characters have to make difficult choices.
Another concept could be a story where the magic in the Harry Potter world and Itachi's ninjutsu are somehow combined. Imagine Itachi using his fire - style jutsu along with a spell from the Harry Potter world to create a unique and powerful attack. This could be used against a common enemy that threatens both the ninja world and the wizarding world.
One interesting idea could be how the themes of sacrifice from Itachi's story in Naruto might be translated into the RWBY universe. For example, Itachi sacrificed his reputation and relationships for the greater good of his village. In RWBY, a character might have to make a similar choice between personal relationships and the safety of the kingdoms. Another aspect could be the combat styles. If Itachi were to exist in the RWBY world, how would his Sharingan abilities blend with the Dust - based combat? It could lead to some really creative fight scenes. Also, the concept of family loyalty, which is strong in Itachi's story, could be explored in relation to the families in RWBY like the Rose or the Schnee families.
One possible title could be 'Itachi Uchiha and the Unseen Bond with the OC'.
Maybe the fanfic could start with the OC being experimented on by Orochimaru. Naruto and Itachi somehow cross paths with the OC. Itachi, seeing the OC's situation, decides to help, which leads to a complex relationship between them all as Naruto tries to understand Itachi's motives and the OC tries to break free from Orochimaru's control.
One plot idea could be that Itachi, in a different timeline where he didn't face all the tragedies so soon, meets Sakura during a mission outside the Leaf Village. They initially start as rivals but gradually come to respect each other's skills. As time passes, they find themselves working together more often and develop a deep friendship that slowly turns into a romantic relationship.
Another plot could be that Itachi has lost his memories and wakes up in the Highschool DxD world. He slowly regains his skills and memories as he experiences the various events in the school. His relationships with the characters in this new world would be very interesting to explore, especially as he starts to remember his past and how it might relate to the new situation.
Another good one is 'Itachi Meets the Avengers'. Here, Itachi's calm and strategic mind makes for some amazing interactions with the Avengers. His abilities seem out of this world to the heroes at first, but they soon realize how useful he can be in battles against powerful foes like Thanos.
Sure. There are often crossovers in fanfiction. Writers like to combine different beloved fictional characters. For the 'Harry Potter is Itachi Uchiha' idea, it could be really interesting. For example, Itachi's Sharingan abilities could be compared or integrated with magic in the Harry Potter universe. It might be a bit of a stretch in terms of the two very different settings, but that's the beauty of fanfiction. You can find such fanfictions on websites like Archive of Our Own or FanFiction.net if they exist.