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.
A plot could revolve around a trainer who has a special connection with both Yveltal and Xerneas. This trainer is caught in the middle of the age - old rivalry between them, and has to find a way to make them understand that they need to coexist peacefully. The trainer could go on a journey to discover the true nature of these two Pokemon and how they are intertwined in the fabric of the Pokemon universe.
Another interesting plot would be a fanfiction set in an alternate universe where Yveltal and Xerneas have switched their roles. Yveltal is now the one associated with life and growth, while Xerneas is linked to destruction. This role - reversal could lead to a lot of internal and external conflicts as the Pokemon themselves struggle with their new identities and the Pokemon world has to adapt to this strange new order.
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.
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 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.
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.
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.
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.
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 the main character being reborn as a Pikachu and having to navigate the world with its electric powers while making friends with other pokemon. Another could be being reborn as a Eevee and having the unique ability to evolve into any form at will, which causes both envy and admiration among other pokemon. Maybe the reborn pokemon has memories of being a human and uses that knowledge to outwit trainers and other threats.
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.