One possible plotline could be Harry using his unique access to Hogwarts to pass false information to Voldemort's followers. For example, he might tell them that a powerful magical artifact is hidden in a certain place, leading them on a wild goose chase while the Order of Phoenix prepares for a real battle.
In the fanfic, Harry might use his status as a 'spy' to get closer to Voldemort's inner circle. He could befriend some of the Death Eaters who are having doubts about Voldemort's leadership. By doing this, he can slowly turn them against Voldemort from within. He might also discover Voldemort's secret plans this way, like a new dark magic spell he is developing or a new target he has in mind.
One interesting plotline could be Voldemort being redeemed. It's a really different take as in the original books he's pure evil. But in fanfic, some writers explore what if he had a change of heart.
Ron could be another important ally. His loyalty and courage would be crucial. He might be the one to act as a decoy sometimes, drawing attention away from Harry when he is on a dangerous spying mission. Also, Ron's ability to blend in with the wizarding crowd could help Harry gather information without being too conspicuous.
In some versions of this fanfic concept, Harry might surrender as part of a long - term strategy. He could be faking it to get closer to Voldemort and find a way to destroy him from the inside. He might endure a lot of hardships and mental torture while pretending to be a submissive prisoner, all the while looking for that one weak spot in Voldemort's defenses that he can exploit to end the Dark Lord once and for all.
One possible plotline could be Harry being held captive in one of Voldemort's dark lairs. Voldemort might use the Cruciatus Curse on him repeatedly to try and break his spirit or get information about the Order of the Phoenix. Harry could be isolated and tortured while his friends desperately search for him.
One interesting plotline could be that Harry discovers a new, powerful magic that gives him an edge in the fight against Voldemort. For example, he might find a way to combine different spells in an unexpected way. Another could be that Voldemort has a secret weakness that Harry uncovers through his friendships. Maybe one of his friends has a unique connection to an ancient magic that can be used against Voldemort.
One possible plotline could be that the children possess unique magical abilities due to their mixed heritage. For example, they might have the power to control both light and dark magic in a balanced way. Another could be that they are caught in the middle of the two worlds, with the wizarding community being suspicious of them because of their Voldemort connection, while Voldemort's remaining followers might try to use them for their own dark purposes.
Well, in some fanfictions, when Harry surrenders to Voldemort, it leads to a strange alliance. Voldemort, being overconfident, might keep Harry alive thinking he can use him. Harry could then start to learn more about the dark side from within, and perhaps find a way to turn some of Voldemort's followers against him. Also, it could be that the act of surrender causes a shift in the magical balance in the wizarding world. Some ancient prophecies might be set in motion that no one expected, and Harry has to figure out how to use this new situation to his advantage.
One possible plotline could be that Voldemort discovers a prophecy that Harry holds the key to some ultimate power that he desires, not just for domination but for something more profound, like the ability to reshape the wizarding world in a way he originally never considered. So he starts to protect Harry instead of trying to kill him immediately.
A possible plotline could be that Voldemort has a change of heart. Maybe he realizes that Harry has some power or knowledge that he needs, so he decides to carry Harry away to a secret location to extract that information from him.
One interesting plotline could be an alternate universe where Harry and Voldemort have to team up against a greater evil. This flips the traditional dynamic on its head.