It could be an ordinary person who has been pushed to the edge. For example, a young man who lost his family to a powerful and corrupt group and decides to take matters into his own hands.
Another possibility is a supernatural being. A half - vampire or a werewolf, for instance, who is shunned by both the human and supernatural communities. This rejection leads them to embrace their dark side and become a killer, using their unique abilities to hunt down their enemies.
Well, it could be about a character who has some sort of tragic backstory that drives them to become a killer. Maybe they were wronged in the past and now seek revenge on those who hurt them.
It could be an aspiring singer or actor. Since the idea of a star being born often relates to the entertainment industry, a character with big dreams in that area makes sense.
It could be an original character created specifically for the fanfiction. Maybe a rogue ninja with a vendetta against the Hidden Leaf Village.
It could be a new character entirely, someone created by the fanfic writer. This character could be an orphan who has a deep connection with Pokemon from a young age.
Sasuke could potentially be the main character in a 'naruto cold killer' fanfic. He already has that brooding and somewhat cold personality. In this fanfiction, he could take his thirst for revenge to an extreme level and become a cold - blooded killer, hunting down those he believes are responsible for his clan's massacre. This would be a very different take on his character compared to the original Naruto story.
A young immigrant might be a good main character. Moving to a new country can be extremely frustrating. There are language barriers, cultural differences, and often discrimination. Their story of trying to build a new life while being born into this frustrating situation of immigration would be very engaging.
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'.
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 human with a special power that makes them a killer could also be a good choice. They could be an outcast in the human world because of their power, and then get drawn into the High School DxD world. Their journey from being a feared killer to finding a place among the supernatural beings would be an exciting story to tell.