One possible plot could be them adjusting to married life in a world filled with the challenges and magic from their respective backgrounds. Maybe they have to face enemies together who are against their union, and they rely on each other's unique skills to survive.
Yes, there might be some out there. You can start by looking on popular fanfiction websites like Archive of Our Own or Fanfiction.net. Search for the specific pairing 'Shirou Emiya' and 'Raven Roth' with the keyword'married' in the search bar, and you should be able to find relevant stories if they exist.
One possible plot could be that Raven Roth is a powerful witch from a far - off land who comes to Hogwarts as an exchange student. She and Harry Potter initially clash but gradually fall in love. After the defeat of Voldemort, they decide to get married and work together to bring about new reforms in the wizarding world, like improving relations between different magical communities.
Well, one possible plot could be that Shirou wakes up one day as a girl and has to deal with all the new physical and social changes. His friends might be really confused at first, like Saber who would be very surprised to see her master in a different form. Shirou, now as a girl, might struggle with things like wearing dresses and having different hormonal emotions. And he would have to face enemies in this new form too, which could lead to some really interesting battle scenes where his opponents underestimate him because he's now a girl.
A plot could involve Shirou being transferred to the Highschool DxD school. He quickly catches the attention of Rias and her peerage. His different style of combat and his unwavering determination might lead to some friendly rivalries within the group. He could also introduce some new concepts of magic that the DxD characters have never seen before, like his ability to reinforce objects. And he may find himself in the middle of the power struggles between the different factions in this new world.
A plot possibility could be that Shirou has to keep his transformation a secret at first. He's afraid of how others will react, especially those he cares about. So he tries to act as normal as possible while dealing with the physical and emotional changes privately. Another could be that some sort of magic gone wrong caused the transformation, and he has to find a way to reverse it while also using his new form to his advantage in battles or other situations.
The plot could revolve around their adventures together as a married couple. Maybe they discover a new magical threat and have to use their combined powers and skills, with Shirou's swordsmanship and Rin's magic, to protect their world. They also might start a family and raise a child with magical abilities, passing on their knowledge and values.
Raven Roth may also be a character who has a mysterious past related to the dark arts. Maybe she was once involved with a group or a situation that had ties to Voldemort, but she has now turned against the dark side. Her knowledge of the dark arts, though dangerous, could be used to help Harry Potter understand his enemies better and find ways to defeat them. This complex backstory would make her a very interesting character in the fanfiction.
There could be. Fanfiction is a realm of infinite possibilities. Since the 'Harry Potter' series has such a large and dedicated fan base, it's very likely that someone has written a story where Harry is married to Raven Roth. You can try searching on popular fanfiction platforms like Archive of Our Own or FanFiction.net.
Perhaps Shirou becomes an Avenger due to a misinterpretation of his ideals. He always wanted to be a hero who saved everyone, but in a world where his actions are constantly misconstrued and lead to more harm than good. He could be driven to the point where he believes that only by taking revenge on those who wronged him and his loved ones can he set things right. This would be a very complex and angsty storyline.
Perhaps they face challenges from the magical world. Since Saber has a connection to the throne of heroes, there could be threats from other entities who don't want a married Saber. They would have to fight together to protect their relationship and their new life. Meanwhile, they are also growing closer and deeper in love as they face these difficulties side by side.