It could be an ordinary person from our world, like a high school student. They're suddenly thrust into this new world and have to adapt. Their inexperience in this new environment can lead to many humorous and exciting situations.
A scientist could be the main character. They use their scientific knowledge to try to understand the new world. For instance, if the new world has different physical laws, they might conduct experiments to figure them out, and their research could also help them gain respect or power in that world.
A young prodigy who has been underestimated all his life. Fed up with being pushed around, he uses his intelligence and unique abilities to rise to power and become the world overlord.
It could be a new student at Hogwarts. This student might come from a non - magical family but has a strong connection to magic that they didn't know about before. Their journey at Hogwarts would be their new beginning at life in the wizarding world.
I think Spider - Man would make an interesting main character. He's young and has a different perspective compared to the more experienced Avengers. His journey of growing up and becoming a more integral part of the team could be a central theme in the fanfiction. He also has his own set of personal problems like school and his relationships which could add depth to the story.
Perhaps a minor character from the original Fate series could be promoted to the main character. For example, a bystander in one of the battles who witnessed something extraordinary and was then drawn into the world of the Holy Grail War in a more central way in the 'a new fate fanfiction'.
Perhaps an old, powerful shinobi who was thought to be dead but has now returned in Naruto's new life. This shinobi could be seeking to reshape the ninja world according to his own dark vision, and Naruto would have to stop him. The antagonist might have some connection to Naruto's past life that is gradually revealed as the story progresses.
It could be a young and ambitious wizard who has come to the town to assert his magical dominance.
Naruto himself could be the main character. Since he's the protagonist of the original series, it would be natural to have him as the center of the fanfiction. He could be drawn into the new game world and have to use his unique skills and determination to succeed.
The main character would most likely be the new thief. They could be an original character created just for this fanfiction.
A former Death Eater who has had a change of heart could be the main character. They are now trying to make amends and start a new life. Their past experiences would make them a complex and interesting character as they struggle to fit into the new, post - Voldemort wizarding society.
The Yonko would definitely be major antagonists. Since Luffy is in the New World, the likes of Kaido or Big Mom would be natural choices. Their vast power and influence would pose a great threat to Luffy and his crew.