A simple alliance in this fanfiction could be between Shiemi Moriyama and Lucy Heartfilia. Shiemi could use her knowledge of plants and herbs, which might be useful in creating potions or antidotes in the Fairy Tail world. Lucy, with her celestial spirits, could provide protection for Shiemi when they encounter demons.
An unlikely alliance could involve Kuro from Blue Exorcist and Happy from Fairy Tail. Kuro is a demon but has a bond with Rin. Happy, on the other hand, is a cat-like Exceed. They could start off as enemies but gradually form an alliance. Maybe they discover a common goal, like finding a lost magical item that is important to both the exorcist world and the Fairy Tail world. This would require them to work together, with Kuro using his demon powers and Happy using his flying ability and knowledge of Fiore.
Well, an unlikely alliance in this fanfiction could be between Ben and Natsu. Natsu is all about fire and brute force, while Ben has his various alien forms with different abilities. They could initially clash due to their different approaches to fighting, but then realize that together they can be much stronger. For instance, if Ben transforms into an alien with wind - based powers, he could enhance Natsu's fire attacks.
Another idea for a Blue Exorcist Demon Tail fanfiction is a story centered around a character who has the powers of both an exorcist and a Fairy Tail mage. This character could be an outcast in both worlds, not fully accepted by the exorcist community because of their magic and not fully trusted by the Fairy Tail guild because of their connection to demons. They would have to find their own path and prove their worth in a world full of prejudice.
A simple alliance in this fanfiction could be that Baiken helps Naruto with her sword skills. Naruto is mainly a ninjutsu user, so her sword - based combat knowledge could be a great addition to his fighting repertoire.
Another aspect in a fanfiction could be their cultural differences. Naruto comes from a world of ninjas with strict honor codes, while Shego is from a more modern - ish world with different moral values. This could lead to many comical or thought - provoking situations. For instance, Naruto might be shocked by Shego's casual attitude towards stealing or using underhanded means in combat. However, as they spend more time together, they begin to see the value in each other's perspectives. They might even influence each other's ways of thinking and acting, with Naruto making Shego more empathetic and Shego teaching Naruto to be a bit more flexible in his approach to problems.
A fanfiction could also explore the idea of Starkiller and Ahsoka meeting in a post - Empire world. Starkiller, having broken free from his Sith past, and Ahsoka, who has seen so much of the galaxy's turmoil, could join forces to help rebuild and protect the newly - emerging peaceful systems. They would have to overcome their differences, as Starkiller has a more aggressive style while Ahsoka is more in tune with the Jedi's peaceful ways. But their shared goal of a better galaxy could bring them together. They could start by training a new generation of Force - sensitive beings, combining their unique teachings to create a more balanced approach to the Force.
In fanfiction, Neji and Sakura could form an unlikely alliance against a common foe within the Hidden Leaf Village. This could be due to some political intrigue where they have to trust each other despite their different personalities.
Another interesting alliance could be between Ron and Keroberos. Ron, with his love for food, might be constantly bribing Keroberos with treats to get information about the Clow Cards or to ask for his help in some comical situations. Keroberos, being a bit of a glutton himself, would be easily swayed by Ron's offerings.
A simple fanfiction plot could be that Peter accidentally stumbles into Wakanda while on a mission. Shuri finds him and at first is suspicious. But then she realizes his good intentions. They start to exchange ideas about technology and hero - work.
In some fanfictions, there could be a time - travel element. What if Draco and Hermione are sent back in time? They might have to work together to ensure the future they know doesn't change. This could lead to some interesting character development as they are forced to rely on each other in a completely different time period, and they may see each other in a new light as they face challenges together.
Perhaps the fanfiction focuses on a parallel universe where Snape, Harry and Lucius are all on the same side from the start. They are part of a secret group that is trying to reform the wizarding world's education system. Their different perspectives - Snape's strictness, Harry's compassion and Lucius' cunning - combine to create a powerful force for change.