Well, a plot could involve Nightmare Rarity getting trapped in a mirror world where everything is opposite to her desires. She has to find a way to escape while also dealing with her own insecurities. Or perhaps she discovers a new magical power that she doesn't fully understand and has to learn to control it without letting it consume her. It could also be that she is on a quest to find a rare gem that has the power to make her even more powerful, but she has to face many challenges along the way.
One plot idea could be Nightmare Rarity trying to take over Equestria's fashion industry with her dark and edgy designs. Another could be her forming an unlikely alliance with other villains to achieve a common goal. Maybe she has a redemption arc where she realizes the error of her ways through the friendship of a kind pony.
One idea could be exploring how Rarity copes with the physical and emotional changes during pregnancy. Maybe she could be designing special maternity wear for herself. Another idea is that her friends in Ponyville come together to support her through this new phase of life. For example, Fluttershy could offer her advice on taking care of herself and the baby.
One common theme is the struggle between good and evil. Since Nightmare Rarity is a darker version, there's often a battle within her or against other characters. Another theme is the power of friendship. Even in a fanfic about a villain - like character, friendship can play a role in her development. Also, the theme of transformation is common, whether it's her changing from Nightmare Rarity back to normal Rarity or evolving into something new.
Well, it could be about Rarity's journey to becoming an alicorn. Maybe she discovers a hidden magic within herself and has to go through a series of trials to transform. For example, she might have to prove her selflessness by using her fashion skills to help those in need in Equestria.
One idea could be a Beezen character in the MLP world having to adapt to the magic there. Maybe they start off as an outsider and gradually learn to use the magic for good, like helping ponies build better hives or something.
One idea could be a wedding between two ponies from different tribes, like a Pegasus and an Earth pony. They would have to overcome the differences in their cultures and traditions. For example, the Pegasus might have a flying ceremony while the Earth pony has a more grounded celebration. Another plot could involve a wedding planner pony trying to create the perfect wedding but facing all sorts of comical disasters along the way, like the flowers not arriving or the cake getting ruined.
One idea could be that Derpy and Dinky go on an adventure in the Everfree Forest. They encounter strange creatures and have to use their wits to get out safely. Another could be a story where Dinky gets lost in Ponyville and Derpy has to search for her, leading to a heartwarming reunion. Or perhaps a plot where they team up to help a new pony in town feel welcome.
One idea could be a story where Celestia tries to redeem Nightmare Moon. Maybe she delves into the past to understand what led her sister to turn into Nightmare Moon in the first place and uses that knowledge to slowly break through the darkness. Another plot could be a time - travel fanfic where a character goes back to prevent Nightmare Moon's creation, and has to work with Celestia to do so. Or perhaps a fanfic where Celestia and Nightmare Moon are forced to team up against a greater evil that threatens Equestria.
In a more adventurous plot, Rarity and Sweetie Belle could be exploring an old, abandoned castle in Equestria. Sweetie Belle is curious and wants to see what treasures might be hidden there. Rarity is a bit more hesitant but goes along to protect her sister. Inside, they find a room full of ancient fabrics and jewels. Rarity is overjoyed at the find, but they also have to deal with some spooky spirits that guard the treasures. Through their combined efforts, they manage to get out safely with some of the treasures and have a great story to tell their friends. This plot would show their different personalities and how they complement each other in difficult situations.
Since 'haunting nightmare mlp fanfic' is a rather broad title, it could cover a variety of themes. It could be centered around a particular pony who is having recurring nightmares. These nightmares might be a sign of some underlying issue in the pony's life, like a past trauma or a present - day problem that they are subconsciously worried about. The story could then follow the pony as they try to figure out the source of these nightmares and how to stop them. It could also involve the other ponies in the community as they support their friend through this difficult time. In the My Little Pony world, friendship is always a key element, so perhaps the power of friendship will be what ultimately helps to dispel the haunting nightmares.
One possible plot could be that Rarity contracts a rare and incurable disease in Equestria. Her friends, like Twilight Sparkle, would try everything to find a cure. But in the end, she passes away peacefully, leaving behind a legacy of her beautiful creations. This could show how the other ponies deal with grief and remember her through her works.