It could be an ex - lover of either of them. This ex - lover might still have feelings and is determined to stop the marriage out of jealousy.
Maybe the local community or some sort of council in the fanfiction setting. They might have rules or traditions that prevent the two from getting married, like differences in social status or caste.
Maybe he has a dark secret. For example, he might be involved in some illegal activities that are yet to be discovered. This would be a big obstacle to their marriage.
Demons could also be the ones to torture Rin. His father is Satan, so other demons might see him as a target. They could torture him to get information about Satan, or perhaps as a way to get revenge on Satan through his son.
Dumbledore, in his mysterious way, could offer some guidance. He might give Harry a riddle or a hint that leads him to his glasses. Also, Luna Lovegood could have some unique insights. She has a different perspective on magic, and she might suggest something completely out - of - the - box, like looking for the glasses in a place where you'd least expect them, like in the Forbidden Forest.
Another possibility is that he marries someone related to Sherlock's world. Maybe a detective or an investigator who he meets during one of Sherlock's cases. They start off as colleagues in solving the mystery and gradually develop feelings for each other.
It could be a jealous demigod. Maybe someone who has always been envious of Percy and Annabeth's relationship and popularity. This person finds a way to manipulate some dark magic or an old rule to force them into marriage, hoping to create chaos in their lives.
He could face social ostracism. Society in that era placed great value on propriety. Forcing a woman into marriage would be seen as improper and could lead to his being shunned by his peers. His reputation would be severely damaged, and he might lose the respect of those around him.
She might use her intelligence. Hermione could try to outwit Draco by finding loopholes in whatever is forcing this marriage, like in the wizarding laws or contracts. For example, she could research ancient magic laws that could nullify such a forced union.
Dumbledore would be a likely candidate. He has a long history with Grindelwald and would not let Harry be in danger at Grindelwald's hands.
Charlotte Pudding could be a likely candidate. She has a complex personality. She's initially deceptive but later shows her true self. Luffy might be attracted to her vulnerability and the good side of her that emerges. And Pudding could be drawn to Luffy's honesty and his ability to see the best in people.
Well, at first it would be really rocky. Naruto might be angry and resentful towards Sasuke for forcing him into this situation. But as time goes on, they could start to understand each other better. Sasuke might realize the harshness of his actions, and Naruto could see that Sasuke had some deeper reasons, perhaps related to protecting Naruto in his own twisted way. Eventually, they might find a way to build a real relationship based on mutual respect and love that grows from their shared experiences.