Another possibility is a new character. Maybe a young ninja from a small village who stumbles upon the dragon armor. This new character could bring a fresh perspective to the Naruto universe. They would have to learn about the armor's powers, deal with the existing Naruto characters who might be interested in the armor, and grow as a ninja throughout the story.
Sasuke could also be a great main character in this fanfiction. Given his drive for power, he might be the one to seek out the dragon armor and use it to further his own goals. He could have an interesting journey of learning to control the power of the armor while also dealing with his inner demons.
It could be a new character created specifically for the fanfiction. Maybe a young ninja who discovers the neglected dragon and forms a bond with it.
Perhaps an anti - hero could be the main character. They find the Infinity Armor and initially use it for their own self - serving purposes. But as the story progresses, they start to see the bigger picture and become more heroic, using the armor to protect not just themselves but also the different worlds they encounter during the crossover.
Sasuke could also be an interesting main character in this type of fanfiction. His cool and brooding nature would contrast well with the majestic and often wild nature of dragons. He could be on a quest to master dragon taming in order to gain more power to seek revenge or protect what he holds dear.
Harry Potter himself could be the main character in this fanfiction. Imagine if he found the basilisk armor and used it to his advantage against Voldemort. It would add an extra layer of protection and power to his fight against the dark lord.
It could be Naruto himself. Imagine Naruto obtaining the powers of the Red and White Dragon Emperor. He would be an even more formidable force, using his ninja skills along with the dragon powers to protect the Hidden Leaf Village.
A young princess could be the main character. She is often seen as a symbol of purity and royalty. The kingdom might be in a dire situation due to the dragon's attacks, so they offer her as a sacrifice. She then has to use her wits and any hidden powers she may have to deal with the dragon, perhaps even trying to change the dragon's nature.
It could be a new character created specifically for this fanfiction. This character could be a young water dragon slayer who gets lost between the two worlds. They would be the main focus as they try to adapt to the Naruto world while still having their Fairy Tail powers.
Sasuke could also be a great choice as the main character in a 'dragon's blood naruto fanfic'. Given his thirst for power and his Sharingan abilities, if he were to obtain the dragon's blood power, it would add a whole new layer to his character. He might use it to seek revenge in a more powerful way or to uncover secrets about his clan that are related to the dragon's blood.
Perhaps a young and inexperienced adventurer who discovers their hidden power while on the quest to slay the dark dragon. They grow throughout the story, learning new skills and becoming a true hero.
An akumatized villain could be the main character in a different kind of fanfiction. The villain was once a normal person but due to some heartbreak or injustice, they are akumatized into a dragon - like form. The story could then be about their journey of redemption or how Ladybug and Chat Noir try to bring them back to their normal self.