One possible plot could be that Remus tries to keep his wizarding world a secret from his Muggle spouse at first. But as time goes on, small magical incidents start to occur around the house, and he has to come clean. Then they face the challenges of the Muggle - wizard divide together.
I'm not sure about specific 'Remus married a Muggle' fanfictions off the top of my head. You could try searching on popular fanfiction websites like Archive of Our Own. There are likely to be some interesting stories there.
One potential Muggle actress could be Carey Mulligan. She has a great range as an actress and could bring a lot of depth to the role of the Muggle wife in the fanfic. Also, Felicity Jones could be a good candidate. She has a charm about her that would fit well with the idea of being Harry's Muggle actress wife. It could be interesting to see how her acting skills would be incorporated into the story, like if she used her acting to help Harry in some undercover wizarding missions in the Muggle world.
One common theme is sacrifice. Remus may have to sacrifice his time with his spouse to fight in the war. Another theme is love in the face of danger. Despite the perils of the war, his love for his partner endures.
I recall a fanfiction titled 'Married in the Midst of War: Remus'. This one is quite detailed. It not only focuses on Remus' marriage during the first war but also weaves in the complex relationships he has with his friends. It shows how his married life changes his perspective on the war and his role in it. The author does an amazing job of creating a believable and immersive world for the characters.
It's popular because Remus and Sirius are both very interesting characters on their own. When you put them together in a marriage, it creates a whole new dynamic. Fans are always eager to see how they interact in different situations as a married couple. And fanfiction writers can explore different aspects of their relationship like how they deal with Remus' lycanthropy or Sirius' family issues within the context of their marriage.
One possible plot twist could be that they keep their marriage a secret from the wizarding world because of the age difference between them. Hermione is much younger than Remus, and the wizarding society might not accept their union easily. Another twist could be that they got married secretly to protect each other. Maybe there's a dark force threatening them, and by being married secretly, they can use some magic spells or charms that are only available to married couples to keep each other safe.
Another possibility is Tom Hanks. Harry could admire his acting skills and all the different characters he has played. He might even compare some of the wizards in his world to the characters Hanks has played, like thinking Dumbledore is a bit like the character in 'Forrest Gump' in terms of being wise and guiding others.
There's 'Remus and Sirius: A Married Life' where the author focuses on their daily routines as a married couple. It shows how they handle family and friends in their relationship. It's really interesting to see how their personalities blend in this new phase of their lives.
Harry would probably be shocked at first. He respects Remus a great deal, but Hermione is like a sister to him. He'd need some time to come to terms with it. Ron, on the other hand, might be more openly opposed. He has always had a bit of a protective streak when it comes to Hermione, and he might think Remus is too old for her. Ginny would likely be more understanding and might try to mediate between the others.