It could be an original character. Maybe a young wizard who discovers a long - lost time - travel spell. This character could be from a family that has been affected by the dark arts in the past and travels back to change their family's fate. They would then get caught up in the larger events of the wizarding world during that time.
Harry himself could be the main character in a different scenario. He might travel back in time to better understand Voldemort's origins. This would give him a chance to find out things he didn't know before and perhaps find a way to completely eradicate the dark magic associated with Voldemort once and for all.
A descendant of one of the original founders of Hogwarts. They might have some special connection or mission related to the school and the wider wizarding world. They would travel around to uncover secrets about their family's past and the history of the magical places.
Scorpius Malfoy is another likely main character. His family's history and his own inquisitive nature would make him an interesting addition. He might be the one who discovers the time - travel mechanism. His relationship with Albus Potter could also be explored further as they embark on this time - traveling adventure together, and his views on his family's past actions could change during the journey.
Another possible antagonist could be some of the Hogwarts faculty who were either secretly supporting Voldemort or were too cowardly to oppose him. In a time - travel revenge fanfic, Harry might uncover their true natures and take revenge on them for not doing more to protect the students and the wizarding world from Voldemort's rise.
Harry Potter himself could be the main character. Imagine him stumbling upon a dark keyblade and having to grapple with its power. He would have to decide whether to use it to fight Voldemort or risk being corrupted by its dark influence.
A young and curious Pokemon trainer would be a great main character. They're always eager to learn and explore, so the concept of time travel would be both exciting and a bit scary for them. They could start off as an ordinary trainer in their own time, but then through some strange event, get sent back or forward in time.
Oscar could be an interesting main character for a time travel RWBY fanfic. As Ozpin's current host, he has a connection to a long history of events in Remnant. Time traveling could allow him to learn more about Ozpin's past directly, and he could also use his new - found knowledge to change things for the better in the present and future of Remnant.
A new, original character could be the main character in a Beyblade time travel fanfic. This character could be a young up - and - coming Beyblader who has a natural talent but is still learning. Time - traveling would be a huge adventure for them, as they would be exposed to different Beyblading cultures and techniques. They could grow and develop their skills in unexpected ways during their time - traveling journey and become a legend in the Beyblading world.
It could be a new character, like a young Auror who has been assigned as an inspector. He would be fresh out of training and eager to prove himself at Hogwarts. He might be a bit naive at first but quickly learns the ropes and uncovers some interesting things about the school.
It could be a young witch who is just starting to explore her magical abilities outside of Hogwarts. She takes on a babysitting job to earn some extra money and ends up having all sorts of magical adventures with the kids she babysits.
A new character could be the main character. For instance, a Saiyan who has been transported to the Harry Potter universe. This character would have to navigate the strange new world of magic while also dealing with their own Saiyan instincts and powers.