One popular theme is the journey of a new trainer in the Sinnoh region, which is the setting for both games. They might face unique challenges and form special bonds with their Pokemon. Another theme could be about legendary Pokemon, like exploring the backstories of Dialga and Palkia in fanfiction. Also, fanfiction often focuses on the rivalries between trainers, adding new twists to the in - game rivalries we see in Platinum and Diamond.
One popular Diamond x Platinum fanfiction could be 'The Shining Alliance'. It often focuses on their unique relationship dynamics and the adventures they might have together. Another might be 'Diamond and Platinum: A Bond Forged', which delves deep into how they first met and how their friendship grew over time. There are also some that explore their roles in a more magical or fantastical setting, like 'Diamond and Platinum in the Enchanted Realm' where they face various magical challenges side by side.
A great storyline might involve a group of friends who each have different goals in the Pokemon Diamond and Platinum world. For example, one might be focused on winning all the gym battles, while another is more interested in finding and documenting rare Pokemon species. Their different paths cross and they end up helping each other in unexpected ways.
Cynthia is also very popular. As the Sinnoh region's champion, she is a powerful and mysterious figure. In fanfics, she can be portrayed as a mentor, a rival, or even an antagonist in some unique storylines. Her Gardevoir and Garchomp are also often featured in these stories.
One idea could be a story where Diamond and Platinum are from rival kingdoms. They meet by chance during a diplomatic event. Their initial animosity turns into curiosity as they discover they have more in common than they thought. Eventually, they work together to solve a problem that threatens both their kingdoms, like a dark magic spell that has been cast upon the land.
One popular character to include is Giratina. Even though it's a Legendary Pokemon, it can be given a more complex personality in fanfiction. Maybe it has a reason for being so feared and banished to the Distortion World that goes beyond what is shown in the games. Also, Fantina, the Gym Leader of Hearthome City, with her unique style and love for Ghost - type Pokemon, can be a great addition to a fanfiction. And don't forget about Maylene, the Fighting - type Gym Leader. Her journey to become stronger and her relationship with her Pokemon can be explored in depth.
One idea could be exploring an alternate universe where the main characters have different Pokemon partners. For example, instead of Dawn having Piplup as her starter, she could start with a Chimchar. Another plot could revolve around a mysterious new Pokemon that only appears in a long - forgotten part of the Sinnoh region. It could be a Pokemon with unique powers that both the good and bad guys are after. Also, a story about a trainer who travels through Sinnoh backwards, starting from the Pokemon League and working their way to the beginning of their journey, facing challenges in reverse order.