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.
It could be a young and inexperienced trainer who is just starting their journey in this new world. They have big dreams but a lot to learn.
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 Pokemon itself. For example, a Pikachu that has been separated from its original trainer and has to fend for itself in the wild. But then it meets a new friend, either a human or another Pokemon, who helps it start over. This Pikachu could be the focus of the story as it overcomes its past traumas and looks forward to a new life.
There's also a possibility that it could be an older character who has been forgotten or underestimated, like Rock Lee. He could have passed on his skills to a new generation but then has to come out of retirement to face a threat that only he can handle in this new dawn of the Naruto world.
It could be a young wizard from a small village who has always dreamed of joining Fairy Tail. This character would be starting a completely new life away from their simple village life, facing the challenges and wonders of the Fairy Tail guild.
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.
It's almost certain that the main character is the lost shinobi. In fanfiction, when the title is structured in this way, the character mentioned first is usually the central figure. This shinobi could be from a well - known ninja clan or be an independent one. Their backstory, such as their training, relationships in their old world, will probably play a role in how they interact with the new world they find themselves in. Maybe they have some special abilities that will help them (or sometimes be a hindrance) in their new situation.
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.