I think Naruto himself would be a likely candidate. His positive attitude and determination are similar to the hobbits' in a way. He's always ready to face challenges, just like the hobbits on their quest to destroy the One Ring. He could use his ninjutsu skills to protect the group from various threats in Middle - earth.
Naruto. He has incredible stamina, his shadow clone jutsu gives him an edge in terms of numbers, and he has the willpower to never give up. He could use his clones to scout the area, set up traps, and then when it comes to a fight, his rasengan would be a very powerful offensive move.
Naruto and the Red Ranger could be a great pairing. Naruto is full of energy and determination, and the Red Ranger is often the leader. They would both lead their respective teams in a combined battle. Another good one could be Sasuke and the Blue Ranger. Sasuke has his cool and calculating nature, similar to the more strategic Blue Ranger.
Naruto himself. He has a way of making friends with almost anyone. His kind heart and determination would likely draw Brynhildr in. He would be interested in her powers and she in his ninja skills, and they could learn a lot from each other, forming a strong bond.
Luke Skywalker could be a likely ally. Luke is known for his open - mindedness and his ability to see the good in people. He would probably be intrigued by Naruto's unique powers and his strong sense of justice. They could team up to fight against the Sith or any other common enemies.
Sasuke is also a strong candidate. Despite their past differences, Sasuke would not let Naruto be held captive by Kumo. He has the power and skills to take on Kumo's ninja if necessary. His determination would drive him to rescue Naruto.
Another possible antagonist could be a rogue ninja from Hailbel's past. This ninja could have a vendetta against her family or something she did in the past. When he discovers that Hailbel is with Naruto, he decides to take them both on. He might have some unique and dangerous techniques that Naruto and Hailbel have to figure out how to counter.
Ninjas, of course! They would come to relax after a long day of training or missions. Also, fans of the ninja world from within the story would be there. Maybe even some of the village leaders would stop by to show support for the local business.
I think Luna Loud would do well in the Harry Potter universe. She has a very creative and open - minded personality, which would be great for exploring the magical world. She might be really interested in Divination or Care of Magical Creatures. And Clyde McBride could also fit in. He's Lincoln's best friend, and his loyalty would make him a great Hufflepuff. He could befriend Neville Longbottom and they could share their experiences of being a bit on the outside at times.
Taylor Hebert might be a strong candidate. Her ability to control bugs gives her a wide - ranging and somewhat unpredictable power. She could potentially use her bugs to disrupt Pennywise's illusions or to find him when he's hiding. Also, her experience with facing difficult situations in the 'Worm' world could give her an edge in dealing with Pennywise.
Sasuke and Stealth Elf. Sasuke is a skilled and somewhat brooding character, and Stealth Elf's stealth abilities would complement Sasuke's own combat skills well. They could work together in a more stealth - based mission in a fanfiction, like infiltrating an enemy base.