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.
Some Pokemon - themed forums also might be a good place. Members often share their own fanfics or recommend ones they've read. For example, the Serebii forums. You can post a thread asking for Xerneas, Yveltal and Zygarde fanfics, and chances are, someone will point you in the right direction.
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.
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.
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.
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.
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.
Some Pokemon - related forums also have sections for fanfics. For example, the Serebii forums. Members often share their own Pokemon Aegislash fanfics there. You might need to register to access all the content, but it's worth it as you can also interact with other fans.
Social media platforms like Reddit can be useful too. There are Pokemon subreddits where fans discuss various aspects of the franchise. You could post a query asking for Garbodor fanfics, and chances are, some fellow fans will point you in the right direction. Also, check out Tumblr as there are many Pokemon fans on there who might have reblogged or created Garbodor - related fanfics.
You can start by looking on major fanfic websites like Wattpad. There are a lot of writers who contribute Pokémon - related fanfics, and you might be lucky to find some great 'female xerneas fanfic' there.
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.