The core tension in a lot of Crowley/Aziraphale stories hinges on their foundational incompatibility. An angel and a demon shouldn't, by cosmic rules, find this kind of companionship. So the central conflict is often external—Heaven and Hell as oppressive institutions—but it gets internalized. Aziraphale's conflict is about his faith versus his lived experience; he loves the world and Crowley, but he's terrified of falling, of being wrong. Crowley's is about hope versus cynicism; he wants to believe in something good (Aziraphale, their side) but six thousand years of disappointment make him brace for betrayal.
What I find most gripping is how fanfic writers map their own anxieties onto this. The fear of being unlovable because of your nature, or the terror that choosing personal happiness means abandoning your duty or community. It's never just 'will they or won't they.' It's 'can they, without one of them being destroyed in the process?' The narratives that stick with me explore that destruction not as a physical thing, but as a loss of self. If Aziraphale chooses Crowley, does he stop being an angel? If Crowley fully accepts Aziraphale's love, does it negate his hard-won, rebellious identity? That's the good stuff.
Lately I've seen more fics grapple directly with the Season 2 finale, turning that emotional conflict into a raw, immediate wound. The trust is shattered, and the question becomes whether their bond is resilient enough to survive not just opposition, but perceived abandonment.