Well, apart from the obvious ones like Link and Zelda, Navi can be a popular character in these fanfictions. Since the reincarnated Link might have a different take on her constant nagging, it can lead to some humorous or deeply emotional interactions. Another popular character could be Sheik. In the original game, Sheik has a mysterious identity which is Zelda in disguise. In fanfictions, the reincarnated Link might figure out this identity in a different way, adding new layers to the story.
A really popular original character in these fanfictions can be an old hermit who lives on the outskirts of the Lost Woods. He has ancient wisdom and gives Link cryptic advice that helps him progress in his quest. There are also characters like a young boy who idolizes Link and tries to follow in his footsteps, adding a bit of humor and a new dynamic to the story.
In some fanfictions of this type, the reincarnated Link might have to deal with the differences in time perception. Since they come from a different world, time might work differently for them. This could lead to some interesting situations like they age at a different rate or they can predict events based on their strange sense of time. They could also have an internal struggle as they try to balance their modern - day morals with the more archaic ones of the Hyrule world.
Ganondorf is a fan - favorite to write about in Ocarina of Time fanfiction. He's the main villain, and there's a lot of room to explore his motives and backstory. Writers can give different interpretations of why he became so power - hungry. Saria is also quite popular. As Link's childhood friend from the Kokiri Forest, she has a special bond with him, and fanfictions often explore the nature of that relationship further, whether it's a platonic friendship or something more.
A plot twist could be that Link, reincarnated as he is, discovers a hidden power within himself that wasn't present in the original Ocarina of Time story. This power could be related to his past life experiences, perhaps some kind of mental strength or a unique magical ability. Another common plot twist might be that the timeline of the game is altered. Instead of following the traditional sequence of events, Link's reincarnation causes ripples that change when certain dungeons are accessible or when key characters make their appearances. Also, there could be a plot twist where Link has to face an unexpected ally turned enemy. A character that was supposed to be on his side in the original story now has a different agenda due to Link's reincarnation.
If Link has modern knowledge, it would definitely shake things up. In terms of communication, he could introduce new ways of sharing information. Maybe he'd come up with a more efficient way to spread news among the different races in Hyrule. His knowledge of basic science could also be applied. He could use the concept of leverage to move heavy objects that were previously thought to be immovable in the game world. Moreover, his understanding of astronomy might lead him to discover new secrets about the night sky in Hyrule, which could be related to some hidden power or location.
Batman is quite popular. Reincarnating as him gives the character a chance to have all his gadgets and detective skills. Also, Superman is a common choice. Having his superpowers and being able to fly around Metropolis would be exciting. Wonder Woman is another one. Her connection to the Amazonian culture and her superhuman abilities make for great fanfiction material.
Another approach is a magical transformation. In the fanfiction world, a powerful magic spell could be cast that turns the protagonist into Link. This could be part of a larger quest, where they need to be Link to save Hyrule from a new threat. For example, a dark sorcerer has cursed the land, and only by becoming Link can the hero access the power of the Ocarina and the Master Sword to defeat the evil.
Veldora is another favorite. His power and his somewhat comical yet powerful nature make him interesting in fanfiction. There could be stories where he takes on a more mentoring role to Rimuru or gets involved in unexpected adventures.
Aether and Lumine are quite popular choices. Since they are the main protagonists in the original game, many fanfiction writers like to explore what it would be like if someone was reincarnated as them, perhaps with different personalities or goals. It gives a fresh perspective on the main storyline.
Jon Snow is quite popular. People like to be reincarnated as him because he has a lot of potential for growth and a mysterious backstory. They can use their new knowledge to perhaps avoid some of his tragedies and rise to power more quickly.