One possible plotline could be that Henrietta, with her own set of skills and experiences, somehow gets transported to the Naruto universe. She might have to adapt to the ninja world, and Naruto could help her in this process while also learning from her in return.
I'm not sure of specific 'naruto x henrietta fanfiction' stories off the top of my head. But you can try looking on fanfiction.net or Archive of Our Own. They usually have a wide variety of fan - made stories for different crossovers.
One common theme is the clash of cultures. Naruto comes from a world of ninjas with its own set of values and Henrietta from a different, often more refined world. Their interactions often explore how they adapt to each other's cultures.
One interesting 'naruto and henrietta fanfiction' could be 'The Meeting of Two Worlds'. It imagines how Naruto, from his ninja world, encounters Henrietta in a completely different setting. The story plays with the idea of their different skills and how they could learn from each other.
First, understand the character of Henrietta well. Know her personality, her backstory, and her relationships in the 'Familiar of Zero' universe. This will help you create a believable interaction with your OC.
Some dedicated anime fan communities also have sections for fanfiction. Check out forums related to 'Familiar of Zero'. Members might share their own Henrietta x OC creations or recommend some great ones they've read.
It's appealing because it allows fans to play with different scenarios. Maybe in one fanfiction, Saito and Henrietta go on an adventure that was not in the original story. It gives a chance to see how their characters would interact in new situations. Also, the fans who love this pairing can create a world where they can be together more often or in different ways than in the main plot. This freedom to create and explore is what makes such fanfictions really interesting.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Familiar of Zero Saito x Henrietta' on their search bars, and you'll likely find a variety of fanfictions related to this pairing.
The theme of sacrifice is also prevalent. Henrietta is in a situation where she may have to sacrifice her own well - being or even her life for the mission. And then there's the theme of friendship. Despite their circumstances, the relationships between Henrietta and the other Gunslinger Girls can be a central theme in fanfiction, showing how they support each other.