A third theme could be the discovery of new powers. When the two universes combine, characters often find themselves with abilities they didn't have before. This could be because of the exposure to the new magic system. It adds an element of excitement and growth to the stories as the characters learn to control and utilize these new powers.
One idea could be that the OC is a new pony in Equestria who has the ability to sense the lingering darkness of Nightmare Moon within Princess Luna. The OC could befriend Luna and help her fully come to terms with that part of her past, leading to a stronger and more confident Luna.
Another aspect could be the impact of Harry on Nightmare Moon herself. His presence might start to soften her more 'nightmare' - like qualities. As he shows kindness and his own brand of heroism, it could lead to a transformation in Nightmare Moon, and they could end up forming an unlikely but strong bond.
Sure. One great 'harry potter and nightmare moon fanfiction' is 'The Magical Encounter'. In this story, the worlds of Harry Potter and Nightmare Moon collide in a really interesting way. It has a unique plot where the characters from both universes have to work together to solve a mystery that threatens both their worlds.
There could also be some new characters introduced in the fanfiction. Maybe a mysterious unicorn with ancient knowledge about Nightmare Moon's origins. Or a group of creatures that have been affected by Nightmare Moon's previous reigns and are now seeking revenge or redemption by helping the ponies stop her third return.
In MLP Nightmare Moon fanfics, the theme of forgiveness can be quite prominent. The other ponies in Equestria may have to decide whether to forgive Nightmare Moon for her past actions. And there's also the theme of self - discovery, as she discovers new aspects of herself during the course of the story.
Well, there's a fanfic that I really liked called 'Nightmare Moon's New Path'. In this story, she teams up with Twilight Sparkle to face a new threat to Equestria. It's interesting to see how their relationship develops throughout the story. There are also some fanfics that explore her relationship with the other princesses in a more in - depth way.
Twilight's relationships with other characters also play a huge role in her development. Her friends' reactions, as mentioned before, can either push her towards self - discovery or make her more withdrawn. If she faces discrimination from some ponies because of her heritage, it could make her more determined to prove herself a force for good.
One possible interesting plot point could be how Twilight discovers her true heritage as Nightmare Moon's daughter. Maybe there are scenes where she starts to exhibit some of Nightmare Moon's powers, like the ability to manipulate the night sky or create illusions related to the moon.