So many of the stories I've stumbled upon focus on the contrast between them, honestly. Glamrock Freddy gets portrayed as the responsible, slightly anxious leader who carries the weight of the world, while Glamrock Bonnie is the flashy, confident performer who seems to have it all together on stage. The tension comes from that dynamic—Bonnie pushing Freddy to loosen up, Freddy grounding Bonnie when the bravado cracks.
A really common thread is exploring the history hinted at in the game. What really happened to Bonnie? A lot of writers go with the angsty 'amnesia after being decommissioned' route, with Freddy being the only one who remembers their past partnership. It's a solid setup for hurt/comfort, with Freddy patiently trying to jog Bonnie's memory or protect him from the truth.
You also see a ton of 'behind the glitter' stories. The idea that under the stage personas and programmed routines, there's this deeper connection the programming can't erase. They'll sneak off after hours to just exist without an audience, or have conversations in their charging stations. It leans hard into the 'more than machines' trope, which works surprisingly well with their designs.