A common idea is to have a character from Supernatural befriend a pony from MLP. For example, Castiel could befriend Twilight Sparkle. They could exchange knowledge, with Castiel sharing his knowledge of the heavens and Twilight sharing her knowledge of magic. And then there could be a story where they have to team up to stop an evil force that is threatening both their worlds, like a powerful demon that wants to drain the magic from Equestria.
Some fanfictions might focus on the moral and ethical differences between the two universes. In Supernatural, there's a lot of grey area in terms of good and evil, while MLP is generally more black and white. So a fanfiction could explore how characters from each would adapt to the other's view on morality. Also, there could be fanfictions about Dean Winchester becoming a pony for a while and having to deal with the day - to - day life in Ponyville, which would be quite a comical situation.
A cool fanfiction idea is a story where a legendary Pokémon ends up in Equestria. For example, Mewtwo could arrive and the ponies initially fear it due to its power. But then they realize Mewtwo is just lost and confused. The ponies, with their kind hearts, try to help Mewtwo find its way back home. Meanwhile, the Pokémon could learn about the magic and friendship in Equestria.
One idea could be that Thomas the Tank Engine somehow ends up in Equestria. Maybe there's a magical portal that transports him there. He could then interact with the ponies, perhaps helping them with some big construction project like building a new bridge. The contrast between his mechanical world and the magical world of MLP would create a lot of comical and interesting situations.
One idea could be that the ponies from MLP end up in the Underground world of Undertale. Maybe Twilight Sparkle could use her magic to interact with the magical elements in Undertale, like the SOULs. Another idea is to have Sans and Papyrus meet Fluttershy. Papyrus would be all excited to show off his 'coolness' while Fluttershy might be a bit shy at first but then they could become friends over their love for cooking (Papyrus loves making spaghetti).
One idea could be that Godzilla somehow ends up in Equestria. The ponies are initially terrified but then realize Godzilla is more interested in protecting the land from some new dark force that has emerged there. Maybe Godzilla's size and power are a mystery to the ponies at first, and Twilight Sparkle tries to study him to figure out how he got there.
Well, in 'mlp meets undertale fanfiction', there could be crossovers of characters. Picture Fluttershy befriending a shy monster in Undertale. Her kind nature could help the monster come out of its shell. Also, the magic system in MLP could blend with the combat or determination - based mechanics in Undertale. It might create new ways for the characters to overcome challenges. For instance, Twilight Sparkle's magic could be used to solve some of the puzzles in the Underground in a different way than the normal Undertale characters would.
One plot idea could be that a group of shadowy villains kidnap Fluttershy because they know of her unique ability to calm and befriend wild creatures. They plan to use her to tame some dangerous magical beasts for their own evil purposes. Another could be that a jealous pony, envious of Fluttershy's popularity in Ponyville, arranges the kidnapping in a bid to make themselves seem more important. Maybe a new, unknown threat from outside Equestria hears about Fluttershy's gentle nature and kidnaps her thinking she'll be an easy target to use as a hostage against the Princesses.
One idea could be a villain OC who has the power to manipulate dreams. They could enter the ponies' dreams and create nightmares, causing chaos and fear. This villain could be motivated by a desire for power over the dream realm.
One idea could be that the Pokemon are discovered in the world of MLP. The ponies could be both amazed and a bit scared at first. For example, Pikachu's electric powers might seem like a strange and powerful magic to them. They would have to figure out how to interact with these new creatures.
One idea could be Naruto characters suddenly finding themselves in the colorful world of MLP. For example, Naruto with his determination might inspire the ponies to face challenges bravely. He could teach them about the power of friendship in a different way, like how he values his bonds with Sasuke and Sakura. Another idea is a pony using Naruto - style jutsu. Picture a unicorn pony using chakra to perform a shadow clone jutsu to get more tasks done around Ponyville.
A good MLP female OC fanfiction idea could be centered around a female OC that is a hybrid of different pony types. Maybe she's part unicorn, part pegasus and part earth pony. She feels like an outcast in Equestria at first because she doesn't fit into any one group. But as the story progresses, she uses her unique combination of abilities to save Equestria from a new threat. Or perhaps a female OC who is a fashion - forward pony in Canterlot. She starts her own fashion line that combines traditional Equestrian styles with new, edgy looks and has to deal with the cut - throat fashion industry in the city.