Another theme could be adventure. They are often put into various dangerous or exciting situations in fanfic, which showcases their courage and the bond between them. For instance, they might embark on a quest to save their world from a mysterious evil force. This not only tests their relationship but also allows for the development of their individual characters in the context of the story.
Well, in the manga, Aoi Tsubaki and Odanna do get married. It's a significant plot point that brings their story to a happy conclusion. The build-up to their marriage is filled with various challenges and sweet moments.
It could be his friends. In many stories, friends are the ones who come to the rescue. They might have some special knowledge or power that can counter the effects of the Deadlights. Maybe they've been researching about it and find a way to break through the hold it has on Richie.
Well, in such a fanfiction, Richie might experience intense psychological horror. The Deadlights are often associated with mind - numbing fear and chaos in the related lore. So, he could be subjected to terrifying visions, lose his grip on reality, and maybe even have his worst nightmares come to life right before his eyes.
One interesting Odanna and Aoi fanfiction could be about their adventures in a new, uncharted part of the spirit world. Maybe they discover a hidden realm full of unique spirits and have to solve a mystery to get back home.