A fanfiction could focus on a mission where Odo and Kira are stranded together on an uninhabited planet. They have to rely on each other to survive. This could explore their survival skills, and also how they deal with being alone together. Maybe they discover new things about each other and their relationship deepens during this time. Or a story about Odo and Kira having to work together to protect DS9 from a new, never - before - seen threat. This threat could be something that challenges their beliefs and forces them to make difficult decisions.
One idea could be a time - travel fanfiction. Odo and Kira get sent back in time to a crucial moment in Bajoran history. They have to use their knowledge from the future to make sure history unfolds correctly, all while trying not to disrupt too much. Another idea is a body - swap fanfic. Odo and Kira accidentally switch forms or minds, and they have to deal with living in each other's shoes for a while. It would be a humorous and also a character - exploring type of story.
How about a fanfiction where Odo and Kira are involved in a diplomatic mission to a planet that has a very different culture from both the Federation and Bajor? They have to navigate this new culture, with its strange customs and politics, all while representing their respective groups. Along the way, they learn more about each other's diplomatic skills and also how they can complement each other in these types of situations. This could also lead to some interesting cultural misunderstandings and how they resolve them.