The idea of them protecting each other is also common. For example, Crowley saving Aziraphale from some angelic danger or vice versa. And then there's the theme of self - discovery. They both have been part of their own 'worlds' for so long, and in fanfics, they often discover new things about themselves through their relationship with the other. It's really interesting to see how different authors play with these themes.
A very popular theme is the 'alternate universe' or AU. In these fanfics, the Good Omens world is reimagined. For instance, they could be in a modern high school setting, where Aziraphale is the nerdy but kind student and Crowley is the cool rebel. Or they could be in a historical setting, like the Victorian era. These AUs allow for a whole new set of adventures and relationship dynamics for Aziraphale and Crowley.
A practical answer is Chapter 1, because it turns the idea into a concrete scene. The chapter opens with Jiang Li, I'm warning you, don't set your sights on Yinwan anymore, or..., adds pressure through Coming to his senses, he also switched to a kindly smile, "Alright, girl,..., and ends on Yet.
Chapter 835, "Is this your way of handling things? So pathetic.", is the clearest early answer because it turns Is's situation into a concrete plot event. A useful checkpoint is the way the scene starts with In a private teahouse in the Capital, a few bejeweled wealthy ladies...
Chapter 731, "Ye Chu Died", is the clearest early answer because it turns Ye Chu Died's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Dean, can I really not do it? and follows through with I heard from the dean you haven't been here in a year..
Chapter 591, "Is There a Way to Get to the Bottom of the Lake?", is the clearest early answer because it turns Fengshui Arrays's situation into a concrete plot event. The chapter gives readers a real answer by opening on But my focus isn't that," Jiang Li's tone shifted, "What exactly is...
Chapter 657, "The Richest Man Becomes a Hardcore Fan?!", is the clearest early answer because it turns Gu Shengrong's situation into a concrete plot event. The chapter gives readers a real answer by opening on Zhao Lang had arrived in the Capital in the early hours of the...
Chapter 758, "What She Says Goes in Our Family!", is the clearest early answer because it turns Jiang Zhao Yuan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Lili was still greeting guests at the front door when suddenly, a...
Chapter 754, "Evil Can Never Defeat Justice", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with After the court trial ended, everyone immediately threw a party for...
Chapter 859, "I Bet She Won't Last Ten Minutes!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 899, "It's actually a seal script?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Cheng, with his head dizzy, was pulled into the courtyard by...