It could be a former Leaf Village ninja who left due to some sort of disagreement. Maybe someone who didn't agree with the village's leadership or its way of training young ninjas.
Well, it depends on the nature of the missing ninja. If the missing ninja has a vendetta against the Hidden Leaf, they might train Naruto for their own malicious purposes. However, if they see potential in Naruto to bring about change, they could train him to be a better ninja in a more morally ambiguous way. Maybe they teach him stealth techniques that are not part of the normal ninja curriculum in the Leaf Village.
The Third Hokage's training might have introduced Naruto to more advanced jutsus. For example, he could have taught Naruto better control over his chakra, which is crucial for performing ninjutsu. This would allow Naruto to use his Rasengan more effectively and with less strain.
One interesting masked ninja is Shinji. He is a masked character in fanfiction who was once a normal ninja but got cursed by an ancient evil. Now, with his masked identity, he tries to find a way to break the curse while also using his new - found dark powers for good in the Naruto world. There's also a masked ninja named Yuki. Yuki is a snow - style user and her mask is made of ice crystals. She is on a quest to find the origin of her ice - based powers. Another is Hikaru, a masked ninja with the power of light, who is often at odds with the darker elements in the Naruto fanfiction world.
One great source for Naruto and Hinata fanfiction could be FanFiction.net. You can search for stories specifically focused on their relationship there. There are often new and interesting takes on their story, from how they first fell in love to their married life in the future.
One possible fanfiction could be about a Naruto ninja who has a special mouth technique. Maybe it allows him to speak different languages and he uses this ability to communicate with various enemies and allies during a mission. He could be on a quest to find an ancient scroll that holds the secrets to enhancing his mouth - related jutsu.
One popular plot idea is a masked ninja being a long - lost relative of a main character. For example, they could be related to Naruto and have been in hiding all this time. Another idea is a masked ninja being a spy from another village, with a complex backstory of loyalty and betrayal. Also, there could be a masked ninja who is a time - traveler, coming from the future to change key events in the Naruto universe.
It could be Orochimaru. He's always been interested in Naruto's power, especially the Nine - Tails within him. He might have come up with a new devious plan to capture Naruto and use his power for his own dark experiments.
Well, Naruto would probably gain some really unique magical abilities. Fairy Tail mages are known for their diverse magic, and Naruto could combine his chakra skills with something like Fire Magic or Celestial Magic. He might become even more powerful in combat, with new ways to attack and defend.
Naruto would gain better combat awareness. Kurama's experience in battles can be passed on to Naruto. He'll be able to read his opponents better, know when to attack and when to defend. This will make him a more formidable opponent in combat situations.