One common theme is the balance between life and death. Xerneas represents life with its life - giving abilities, and Yveltal represents destruction or death. Fanfics often explore how these two forces interact and keep a balance in the Pokemon universe.
One idea could be a story where Yveltal and Xerneas are forced to team up to face a new, greater threat that endangers their entire Pokemon world. Maybe a powerful and evil force is spreading chaos and they realize they can't defeat it alone.
One idea could be a story where Xerneas and Yveltal have to team up to save the Pokemon world from a new, unknown threat. They have different powers and personalities, so it would be full of conflict and cooperation.
The plot is crucial. It could be a story about the struggle between Xerneas and Yveltal's opposing powers and how Zygarde tries to maintain the balance. Or perhaps a story where the trainer has to deal with the consequences of the actions of these powerful Pokemon. A plot that keeps you on your toes and makes you want to keep reading is a sign of a great fanfic.
Xerneas, on the other hand, is all about rejuvenation. Its power can be shown as a bright, glowing light that spreads across the land. If a barren field is in the path of Xerneas' power, it will soon be filled with lush grass and blooming flowers. In a fanfiction, a character who is injured or sick might be healed by just being near Xerneas.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. There are often many talented writers who create stories about these Pokemon. Just search for 'Xerneas Yveltal Zygarde fanfic' in the search bar, and you'll get a list of relevant stories.
There's also a fanfic called 'Xerneas' Glow and Yveltal's Shadow'. In this story, it delves into the symbolic meaning of Xerneas' life - giving light and Yveltal's more destructive nature. It has an interesting plot that unfolds as the two Pokemon cross paths in a series of adventures.
The relationship can be explored through battles. For example, a fanfic could show a battle between them where they both realize they are evenly matched and start to respect each other's power.
One idea could be a story where Xerneas and Yveltal are forced to cooperate in a world where their powers are needed to stop a greater evil. Their different natures, Xerneas representing life and Yveltal representing destruction, create an interesting dynamic as they learn to work together. Another plot could involve a trainer who has a special bond with both Xerneas and Yveltal and has to navigate the challenges of their opposing forces in a battle tournament.
One great Pokemon Yveltal fanfiction is 'The Awakening of Yveltal'. It delves into the mystery of Yveltal's origin and its connection to the other legendary Pokemon. The author does an amazing job of describing Yveltal's power and its role in the Pokemon world.