The '12 life' concept might be a metaphorical way of looking at Harry's life. It could represent 12 different stages of his growth and development. In the first 'life', he could be the young, naive boy we first meet in the books. As the 'lives' progress, he becomes more mature, more powerful, and faces more complex problems. It could also involve him interacting with different versions of his friends and foes from these various 'lives', creating a really rich and complex narrative.
It could be about Harry experiencing 12 different lifetimes, each with its own set of challenges and adventures. Maybe in one life he's a powerful dark wizard, in another a humble healer, and so on.
Perhaps it's a fanfic that takes the concept of Persephone's abduction in Greek mythology and applies it to the Harry Potter universe. The 'his' could be a dark wizard who kidnaps Persephone, a powerful witch, and the plot could be about her friends in Hogwarts trying to rescue her. Or it could be a more romantic take, where 'his' is a hero - type character and Persephone is in some sort of danger related to the ongoing battle between good and evil in the wizarding world, and he has to save her.
The plot could involve Harry being a reincarnation of Anubis. He might start to have strange dreams and visions related to his past life as the Egyptian god. As he tries to understand these, he discovers that his powers as Anubis are starting to surface. This could lead to him having to protect the wizarding world from a new kind of threat that only his combined powers can defeat.
Maybe Harry Potter and his friends have to follow Colonello's orders for some mission in the wizarding world.
Maybe Gandalf appears in the wizarding world of Harry Potter and helps him to defeat Voldemort. He could use his powerful magic that is different from what they have in Hogwarts.
One way is that Harry discovers a spell that allows him to communicate with the dead. Using this spell, he contacts his parents. They give him advice on how to defeat Voldemort, like a secret weakness of the Dark Lord.
In this fanfiction, perhaps the Japanese god has a prophecy about Harry. The plot might be that Harry is transported to a realm where Japanese mythology and magic coexist. There, he has to prove himself worthy of the god's power. He could learn new forms of magic, make new friends from the Japanese mythological creatures, and at the same time, figure out how to use this new power to protect his own world. For instance, he might have to face a dark wizard who has also tapped into some of the power of the Japanese mythological world, and Harry's newfound connection to the god gives him an edge in the battle.
Perhaps the plot could involve the Hogwarts houses competing like 'Big Brother' contestants. Each house has to strategize, form alliances, and complete tasks to avoid being 'evicted'. Harry could be a key player in Gryffindor's strategy.
Well, one possible plot could be that Harry uses his magic to combat various threats to the country that normal politicians can't handle. Like using spells to stop terrorist attacks or natural disasters. He might also have to hide his magic from the public while still using it for the greater good.
One possible plot could be that Harry discovers a hidden connection between his wizarding world and the world of vampires. Dracula, for some reason, sees potential in Harry and decides to make him his'mate', which could mean an apprentice or a partner in some dark magic deeds. Harry, initially reluctant, gets drawn into Dracula's world as he realizes there are secrets about his own magic that Dracula can help him uncover.
One possible plot could be that the fem Harry Potter discovers she has a hidden connection to Mordred. Maybe she starts having strange dreams about the Arthurian era and slowly uncovers that she has some of Mordred's essence or powers within her. This could lead to her having to deal with new magical threats that are related to the Arthurian legends, while also still facing the usual challenges in the Harry Potter world like dealing with Voldemort or his followers.