One idea could be exploring Ori's relationship with a new character in a romantic and somewhat adventurous way. Maybe Ori discovers a hidden part of the forest with this new character and they share special moments there.
Well, you could write a fan fiction where Ori has to face a new threat that is not related to the decay. For example, a group of magical beings from outside the forest come to claim the forest's magic for themselves. Ori then has to team up with some of the forest's most unlikely allies, like the silent spiders, to drive them away. Or perhaps Ori gets lost in a time - loop and has to figure out how to break it while also saving the forest from a sudden blight that is spreading faster than ever.
One idea could be a crossover where Naruto's skills are combined with Ori's abilities in the Blind Forest. For example, Naruto could use his shadow clone jutsu to help Ori solve puzzles or fight off enemies more efficiently.
One idea could be exploring Ori's relationship with other characters in more depth. For example, how Ori's friendship with Gumo might develop further. Maybe they could go on an adventure together outside of the forest they know.
You can focus on the unique aspects of the game's world. Like, describe in great detail the magical elements of the Blind Forest that play a role in the relationship. For instance, if Ori and another character are sharing a moment near a special tree that has some sort of power, that can add a unique touch.
Naru is a great character to include. She has a strong connection to Ori and her backstory can be further explored. For example, you could write about her younger days in the forest. Sein is also important. You could write a story where Sein's power malfunctions and Ori has to help her regain control. And Gumo, the strange spider - like creature, can add an interesting dynamic. Maybe he has a secret that only Ori can help him with.
One idea could be that the two characters who go on a blind date have completely different expectations. For example, one might be looking for a serious relationship while the other just wants a fun night out. Then, as they start talking, they discover they have an unexpected common interest, like a love for a really obscure band. This could lead to a series of humorous and heartwarming exchanges.
One idea could be that Sasuke and Sakura are on a mission in the forest. They get separated from their team and have to rely on each other to survive. Along the way, they start to open up to one another about their feelings, which they've been suppressing for a long time.
One idea could be a story where Steven, despite being blind, discovers a new power within himself that is related to his sense of touch. For example, he can sense the emotions of the Gems through their gemstones when he touches them. This would add a new dimension to his relationships with the other characters.
A forest fanfic could also center around a forest spirit. This spirit has the ability to shape - shift into different forms. A human stumbles upon the forest and forms an unlikely bond with the spirit. Together, they face threats from loggers who want to destroy the forest, and they fight to protect it.
First, you should replay the game to get a fresh feel for the world, the characters, and the story. This will help you come up with ideas that fit well within the existing lore.