One possibility could be that there was some sort of magical interference. Maybe a powerful magical artifact or a spell was in place that deflected the Killing Curse just enough for James to survive. For example, if there was a hidden protection charm on him that he wasn't aware of.
One interesting plot could be that James, having survived, decides to go into hiding with his family in a more secretive way. He uses his survival as a motivation to become a stronger wizard and starts to train secretly, waiting for the right moment to face Voldemort again.
There might be. Fanfiction writers sometimes like to take characters out of their comfort zones and explore darker or more extreme scenarios. But in the official 'Harry Potter' lore, Harry would never casually use the Killing Curse. It's a very serious and immoral spell in the wizarding world.
If Harry uses the Killing Curse in fanfiction, it would greatly change the perception of his character. He has always been seen as a hero who stands against dark magic, so using such a curse would make him seem more morally ambiguous.
In some fanfictions, if they survived, they might have changed the whole course of the wizarding war. They could have joined forces with other powerful wizards and witches earlier and potentially prevented Voldemort from gaining as much power as he did in the original story. They might have also influenced the Hogwarts curriculum, emphasizing more on defensive magic against the Dark Arts from an earlier stage.
Well, a great James Potter survived fanfiction is 'James Potter's Redemption Arc'. It could explore how his survival changes his perspective on life and death. Maybe he has to deal with the guilt of leaving his family behind in the first place. Then there's 'The Return of James Potter'. This one might show his struggles to fit back into the wizarding world he once knew, as so much has changed since his supposed death.
In some fanfiction, the immediate consequence of the Killing Curse is not always death. There could be a situation where the victim's soul is split, like what happened to Voldemort when he tried to kill Harry as a baby. This creates a complex situation where the victim may have a part of their being that is still 'alive' in a strange way.
Perhaps James had some hidden magical ability that he was unaware of until that moment. It could be something that was passed down through his family but had never manifested before. This ability could have protected him from Voldemort's killing curse. Or, it's possible that there was a plot by some other characters to save James. There could have been a group of wizards or witches who had been secretly working to keep James alive because they knew his importance in the fight against Voldemort. They might have used some sort of time - turning magic or a protective charm that Voldemort couldn't break through.
The relationship between Harry and his parents would be completely different. He would grow up with their love and guidance, rather than feeling like an orphan. There would be a much stronger family bond.
Another plot might involve a character other than Voldemort using the Killing Curse in a moment of extreme desperation. For example, a Hogwarts student who is being blackmailed by Death Eaters and uses it to defend themselves, but then has to deal with the moral and magical consequences. It could show the dark side of the wizarding world even among the so - called 'good' characters.