One common theme is friendship. Their relationship often starts as a simple acquaintance and then develops into a deep friendship that they rely on in difficult times. For example, in many fanfictions, they help each other out during battles or challenges.
One great baki oliver x doyle fanfiction is 'The Unlikely Bond'. It beautifully explores their relationship in a world full of adventure. The author does a great job of bringing out their personalities and how they interact with each other.