A common theme is friendship. Given their different backgrounds - Bastion being a reformed war machine and Orisa being a protector - their friendship can be really interesting to explore. They learn from each other.
You can also check out FanFiction.net. There might be some interesting Bastion x Orisa stories. Maybe there are some that take a more adventurous route, like they go on a mission together outside of their normal Overwatch duties. It could be a story where they have to face some new threat that neither of them has encountered before and they rely on each other's strengths to overcome it.
One common theme is friendship development. Since they are both part of the Overwatch universe, stories often explore how they become friends and support each other in their respective roles. For example, Bastion, with its combat capabilities, and Orisa, with her defensive skills, might team up in a story to face a common threat.
Fanfiction.net might also be a good place. There are many Overwatch fanfics there. However, the quality can vary. Some stories might be really well - written while others not so much. But it's definitely worth a search for Bastion x Orisa fanfictions.
One great 'Orisa x Bastion fanfic' is 'The Unlikely Bond' which focuses on their unique friendship within the Overwatch universe. It shows how they support each other during different missions.
One common theme is friendship. Bastion, being a rather unique character in the Overwatch universe, often forms unexpected friendships in fanfiction, especially with other heroes. Another theme is redemption. Bastion's past as a war machine can lead to stories where it tries to atone for its actions. And of course, there are also romantic themes in some of the fanfictions, exploring a relationship between Bastion and another character.
One possible plot could be that Bastion and Orisa are on a mission to protect a certain area from enemy invaders. They start off as comrades just following orders, but as they face various challenges together, they develop a deeper understanding and friendship. For example, Bastion's combat skills and Orisa's defensive capabilities complement each other during battles. They might also encounter some local wildlife that Bastion is curious about, while Orisa is more focused on the strategic aspect of the mission.
One great Overwatch Bastion crossover fanfiction is 'Bastion in Another World'. It has Bastion being transported to a fantasy realm. The author does an amazing job of describing how Bastion's mechanical nature interacts with the magic of the new world.
Well, Tumblr can be a great place to find Bastion x Overwatch fanfiction. A lot of fans post their works there, and you can search using relevant tags. Additionally, Reddit has some subreddits dedicated to Overwatch fanfiction. Just be sure to read the rules before posting or asking for more stories.