A young and inexperienced member of the vanguard could also be a main character. Their naivete in dealing with the past could create interesting situations and conflicts. For example, they might accidentally change a small detail that has big consequences.
Obviously, Kaneki himself would be the central character. But also, his friends like Hide could be involved. Hide was always there for Kaneki, and if Kaneki goes back in time, he might want to protect Hide better from the start. Touka could also be a major part. Maybe Kaneki goes back to a time when he first met Touka and tries to change their relationship for the better, and together they could work towards changing the fate of ghouls.
In 'Desmond goes back in time fanfiction', Desmond is surely a main character. He could meet his own family members from the past, and they could be important characters too. For instance, his great - grandfather might be a key character. There could also be a love interest for Desmond in the past, a local girl perhaps, who adds an element of romance to the story and also influences his actions and decisions while he's in the past.
There could also be characters from the past that become central to the story. For example, if they go back to the time of the First Hokage, Hashirama Senju could be a major character. Team 7 might interact with him, learn from him, or even have to work with or against him depending on the plot of the fanfiction.
Inuyasha could be one of the main characters. Since he has a strong connection with Kagome in the original story, he might also be involved in the time - traveling adventure. Maybe he follows Kagome back in time or is already in the past waiting for her.
Perhaps Orochimaru, if he somehow discovers Ino's time - traveling. He has always been interested in power and forbidden jutsu. He might try to use Ino's ability for his own gain, and if she resists, he would become an antagonist. He could also try to stop her from changing things that he wants to keep as they are for his own evil purposes.
It could be a time - traveling version of Orochimaru. He might have his own plans for the past and Sasuke's interference could put them at risk. So, he tries to stop Sasuke from changing the past in a way that would undermine his future goals.
It could be Danzo. In the original story, Danzo was a major factor in the Uchiha massacre. If Itachi goes back in time, Danzo would likely still be plotting against the Uchiha. His thirst for power and his extreme methods would put him at odds with Itachi's new attempts to save the clan.
It could be Zeref. If Natsu goes back in time, Zeref might have different plans or be in a more powerful position at that time. He could be the main obstacle for Natsu to overcome, especially if Natsu is trying to change things related to the dark magic that Zeref wields.
It could be a time - traveling version of one of his existing villains. For example, Zoom could have found a way to travel back in time and is trying to change history to make his own future more powerful. Another option is that a new character, a time - keeper type of villain, emerges. This villain's goal is to stop Barry from messing with the timeline and will do anything to preserve the original course of history, even if it means harming Barry.
It could be a powerful sorcerer from the time she travels back to. This sorcerer might sense that Elena doesn't belong in that time and wants to stop her from interfering with his plans. Maybe he has some dark magic he wants to use to take over the kingdom or town she's in.