The year in which the Witch Hunters novel is set can vary depending on the author's imagination and the story's context. It could be a fictional or historical setting. You might find clues or mentions throughout the story to determine the exact year.
Maybe the OC in the fanfiction is a witch herself but wants to help Hansel and Gretel fight the bad witches. She has to hide her identity at first. As they hunt witches, she gradually gains their trust by using her knowledge of witchcraft to outwit the evil ones. And in the end, they all form a strong alliance.
A good OC could be a magical healer. Since witch hunting is dangerous, having someone who can heal their wounds would be very useful for Hansel and Gretel. This healer could also have some knowledge of potions that can counteract the spells of witches.