Well, one interesting plot could be the swordsman character having to protect his harem members from various threats in the Naruto world, like rogue ninjas. Another could be the development of relationships within the harem, with different members having unique personalities and backstories that interact with the swordsman's own journey.
Kakashi could be a potential swordsman character. He's known for his combat skills. In the harem, Hinata could be there. Her gentle nature would contrast with the action-packed world. Also, Ino could be part of it, using her mind - control jutsu in unique ways to protect the group and be involved in the harem's relationships.
One possible fanfic could be about a character who wields a unique sword with special powers. Maybe it's a hidden clan member who has been training in secret all this time and emerges as the strongest swordsman. They could use their sword skills to fight against powerful enemies like the Akatsuki members, adding a new and exciting dynamic to the Naruto universe.
It could be Sasuke. He already has great combat skills and if he were to focus more on swordsmanship, he could be a very strong swordsman. His Sharingan would give him an edge in predicting his opponents' moves and countering them effectively with his sword.
In a 'naruto is in a au harem fanfic', aside from the obvious choices like Sakura and Hinata, characters like Temari could be in Naruto's harem. She's from another village and has a strong personality. Her presence would bring in an element of cross - village relations. Karui is also a possibility. She's feisty and has her own set of skills. And then there's Kurenai, the jonin. She could be an interesting addition to the harem, with her experience and calm demeanor.
One common theme is power - up. The swordsman characters usually start with some basic sword skills and gradually become extremely powerful. Another theme is friendship. They often form strong bonds with Naruto characters while on their adventures.
One possible plot is that Naruto is sent on a special mission where he encounters a powerful swordsman. This swordsman becomes his mentor and teaches him the ways of the sword. Naruto, with his natural determination and the power of the Nine - Tails, quickly masters the art and becomes a godlike swordsman. He then uses his new - found skills to face off against powerful enemies that threaten the ninja world, all while changing the perception of what a ninja can be.
There might also be a mentor figure, someone who has taught the ultimate swordsman their skills. This character could be an old and wise ninja with a lot of experience in swordsmanship.
Sasuke could be a great main character for a swordsman - themed fanfic. Since he already uses a sword - like weapon, the story could focus on his further development as a swordsman, exploring different sword techniques and their origins.
Well, one common element could be the use of unique powers. In Naruto, there are ninjutsu and chakra-based abilities. In Fairy Tail, magic is the main power source. A swordsman in this fanfic might combine the precision of swordsmanship with these powers. For example, a character could use chakra or magic to enhance the strength and speed of sword strikes. Another element could be the team dynamics. Just like in the original series, characters from Naruto and Fairy Tail might form teams and learn to work together, with the swordsman perhaps being a key fighter in the group, bringing a different combat style to the mix.