In this type of fanfiction, a plot twist could be that Snape saves them, but they end up having to go into hiding in a completely unexpected place. Maybe they have to live among muggles in a way they never thought possible. Another plot twist could be that Snape saves them, but then realizes that he has put himself in a very dangerous position with Voldemort's followers. So he has to find a way to protect himself while also protecting Lily and James. And there could also be a plot twist where Snape saves them, but they don't believe his motives are pure and they try to escape from him, leading to a whole new set of adventures and challenges.
In some versions, James might use his Quidditch skills to create a distraction while rescuing Lily from a dangerous situation Snape unknowingly led her into. For example, he could fly in on his broomstick and snatch her away just as a spell was about to hit her.
One common plot twist is when Lily discovers Snape's deeper involvement with the Dark Arts later than expected. This realization makes her lash out at him more harshly than before, causing great pain. For instance, she might find out about some of his secret experiments or associations.
One possible plot twist could be that Lily discovers she has some sort of magical complication during the pregnancy that endangers her life or the baby's. This would add a lot of drama and tension to the story.
James is likely portrayed as brave and heroic. He is willing to put himself in danger to save Lily, showing his chivalrous side. He probably also has a bit of a cocky streak as he is confident in his ability to rescue her.
One possible plot twist could be that Lily marries Snape out of a sense of duty. Maybe she discovers some long - hidden secret about Snape's past that makes her feel obligated to stand by him. For example, if she finds out that he made a great sacrifice for her family in the past that no one else knew about. Another plot twist could be that it's a marriage of convenience. The wizarding world is in danger, and their union is seen as a way to bring two powerful magical lineages together to fight against a common enemy. Or perhaps Snape has some unique magical ability that Lily needs in order to achieve a certain goal, and marriage is the only way to access it.
One reason could be jealousy. Snape might be jealous of James' popularity and charm. James, on the other hand, could be jealous of Snape's close friendship with Lily in their early years. Another reason is their different personalities. James was outgoing and bold, while Snape was more introverted and brooding. Lily might be attracted to different aspects of each of them, which could lead to the fight as they both compete for her attention.
A really interesting plot twist could be that after getting pregnant, Lily loses her memory. So Snape has to deal with not only the fact that he's going to be a father but also that Lily doesn't remember their relationship. And then James steps in to help, thinking the baby is his. This would create a very strange love triangle situation. Also, what if the baby Harry has some sort of special power from the start that no one expected, which could be a major plot twist.
One possible plot twist could be that the real danger was not what it seemed at first. For example, while the Avengers thought they were being attacked by a supervillain, it was actually a test set up by a greater power. And Peter discovers this truth while trying to save them. Another twist could be that Peter has to sacrifice one of his powers temporarily to save the Avengers, which changes his approach to being a superhero.
Perhaps the twist is that Sirius saves Draco, but in doing so, he sacrifices his own freedom. And then Draco has to figure out a way to return the favor and free Sirius. Or it could be that the reason Sirius saves Draco is not what it seems at first. Instead of pure altruism, he has a hidden motive related to his own past or a prophecy that involves Draco.
In some fanfictions, a plot twist could be that Voldemort tries to attack during the wedding. This adds a whole new level of danger and drama to the story. It forces James and Lily to rely on their friends and their own magical abilities to protect everyone at the wedding.