Emiya Archer from Fate Stay Night also has a lot of potential in this crossover. His Unlimited Blade Works is a formidable ability. He could potentially use it to counter many of the strange powers in the Re Zero world. And his experience and strategic mind would make him a tough opponent for any character from either universe.
I think Reinhard van Astrea from Re Zero could also be very powerful in a crossover. His incredible strength and combat abilities are on par with some of the strongest Servants in Fate. If he were to face off against them, it would be a very intense battle, and his loyalty and sense of justice might give him an extra boost in a fight.
It's hard to say for sure, but Saber from Fate Stay Night would be a very strong contender. Her combat skills and noble phantasm are extremely powerful. However, in the context of a crossover, Subaru's Return by Death ability could potentially give him an edge. He could keep trying different strategies against her until he found a way to defeat her.
It could be Albus Dumbledore. He has great magical knowledge and power in the 'Harry Potter' universe. When combined with the 'Fate Stay Night' elements, he might be able to outwit even the most powerful Servants. His ability to plan and his understanding of magic could give him an edge in any conflict.
Well, if we consider Harry Potter himself, he has the power of love and the ability to face great evil. But when compared to some of the Servants in 'Fate Stay Night', he might seem less powerful at first glance. But his determination and the fact that he has overcome death multiple times could give him an advantage. On the other hand, a character like Archer from 'Fate Stay Night' has a wide range of skills and the ability to analyze situations quickly. In a combined fanfiction, it would depend a lot on the situation and how the powers interact. For example, if it's a battle of wits, Dumbledore or Archer might have the upper hand, but if it's a battle of pure power, Gilgamesh or a magically enhanced Harry Potter could be the victor.
It's really hard to say who would be the most powerful. On one hand, in Fate Stay Night, Servants like Gilgamesh have incredible power. His Gate of Babylon contains countless noble phantasms. But in the Naruto world, characters like Naruto in his Six Paths Sage Mode are also extremely powerful. He has the power of the tailed beasts and can use all kinds of powerful jutsu. However, if we consider a crossover, it might be someone who can combine the powers from both worlds. For example, if a mage from Fate could learn to use chakra and combine it with their magic, they could be very powerful. Or if Naruto could somehow gain the abilities of a Servant, like the ability to use noble phantasms, he would be unstoppable.
Another great combination could be Archer from Fate Stay Night and Kazuya from Freezing. Archer's cynical and experienced nature would contrast well with Kazuya's more naive and kind - hearted personality. In a battle situation, Archer's long - range archery skills combined with Kazuya's support abilities for the Pandora could create some really cool combat scenes. Plus, their different outlooks on life would lead to some interesting philosophical discussions between them.
Saber would be really interesting. Her noble and chivalrous nature would contrast with the rough - and - tumble world of Skyrim. She has a strong sense of justice and honor, which could put her at odds with some of the more morally gray characters in Skyrim. Her swordsmanship skills would be a great asset in a land full of combat, and her magical abilities could be a mystery to the mages of Skyrim. Also, her backstory as a king in her own world could lead to some really interesting interactions with the Jarls and other leaders in Skyrim.
One of the most interesting pairings would be Saber and Elsa. They are both strong female characters with powerful magic. Saber could teach Elsa about combat and leadership, while Elsa could show Saber how to better control her magic in a more natural way. Also, Kristoff and Lancer could be an unexpected but interesting pairing. Lancer's pride and Kristoff's down - to - earth nature would create some fun interactions.
Perhaps Shirou from Fate Stay Night could find himself in the Re Zero world. He might use his skills as a mage and his determination to help Subaru in his quests. Their different outlooks on life and combat could lead to some interesting character development as they learn from each other.
I think Harry Potter and Saber would make an interesting pairing. Harry has his own sense of justice and courage, and Saber is a noble and powerful Servant. They could learn from each other's leadership styles. Another good pairing could be Hermione and Rin. Both are intelligent and skilled in magic. They could exchange knowledge about their different magical worlds and maybe even come up with new spells or magical techniques together.
It could be Naruto. His ability to use chakra and his vast array of jutsus are very powerful. In the Fate universe, he might be able to combine his chakra with the magic there. For example, he could infuse his Rasengan with magical energy, making it even more destructive.
Merlin himself is often a very popular character. His powerful magic and mysterious nature make him a focal point. Writers like to explore his abilities further and put him in different scenarios.