Ever notice how the best Constantine stories sidestep the tired old 'Chosen One' destiny stuff? The stuff I keep coming back to is when he's just a stubborn, chain-smoking bastard stuck in a system that's already broken. There's this one trope I'm a sucker for: Constantine inheriting a problem, or a debt, from another character. Not some grand prophecy, but something messy and personal left behind by an old 'acquaintance' who's now dead or missing. It immediately grounds the magic in consequence. He's not fighting for the fate of the world; he's cleaning up a specific, haunted mess, which always feels more true to the character's cynical roots.
I also think the 'failed exorcism' is criminally underused. Most fics have him waltz in, say a few Latin words, and banish the demon. But the real juice is in the aftermath when the ritual doesn't work, or works in a way he didn't anticipate. The house is still haunted, but now the ghost is angry at him specifically. Or the demon's bound, but to an object in the room, not back to Hell. Those stories force him to be clever, not just powerful, and they highlight the inherent unpredictability he's always warning people about. They make the magic feel dangerous again, something that can bite the hand that tries to wield it.
Finally, I'm a big fan of fics that explore the mundane collateral damage. The focus isn't on the spectral showdown, but on the water damage in the client's basement from the salt circles, or the landlord threatening eviction because of the sulfur smell, or John having to sweet-talk a cop who found a weird symbol carved into a crime scene. That grimy, street-level consequence is what separates a Constantine tale from a generic supernatural thriller. It reminds you that magic has a cost, and the bill always comes due, usually from the most inconvenient angle possible.