One idea could be to explore an alternate storyline where the main character fails to stop Team Galactic's initial plan and has to deal with the consequences. Another could be a fanfic centered around a new Pokemon species discovered only in the Platinum region that has unique abilities. Or perhaps a story about a trainer who has the ability to communicate with Pokemon in a more in - depth way than normal trainers and how that affects their journey in the Pokemon Platinum world.
First, do your research on Pokemon Platinum and Darkrai. Know their characteristics, abilities, and the lore surrounding them. For Darkrai, it's important to understand its dream - related powers. Then, come up with a unique plot. Don't just copy existing stories. Maybe think about how Darkrai would interact with other Pokemon or trainers in the Platinum world in a new way.
The main character from the game, of course, is a popular choice. Also, Cyrus from Team Galactic is often included as he is such a complex and interesting villain. And Dawn or Lucas, depending on the version, are great to have as they are the typical Pokemon trainers that players can relate to.
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 plot idea could be Darkrai's struggle to fit into the Pokemon world of Pokemon Platinum. Since it's often seen as a menacing and mysterious Pokemon, it could be about it trying to prove that it has good intentions. For example, it might try to protect a small village from other powerful Pokemon threats, all while the villagers are initially afraid of it.
Team Galactic's Cyrus. He is a complex villain with a grand plan. A fanfic could delve into his motives more deeply, or perhaps show a different side of him, like what led him to his extreme beliefs in the first place. His interactions with the main characters could also be a major part of the story.
Maybe a fanfic where a trainer's Pokemon gets a strange and unique ability due to an encounter with a mysterious Pokemon in the Platinum and Pearl world. This could lead to all sorts of adventures as they try to understand and control this new power.
There are fanfictions that focus on the backstories of the Legendary Pokemon in Pokemon Platinum. These might explore the origin of Dialga and Palkia, their relationship, and how they interact with the world of Sinnoh. It could include things like their role in the creation of time and space and how trainers might encounter them in different, more in - depth ways.
The Old Chateau in Pokemon Platinum has a spooky story. It's an old, abandoned mansion. There are rumors of ghosts haunting it. You can encounter strange Pokemon there like Gastly and Haunter. It's a place full of mystery and gives an eerie feeling when you explore it in the game.