One possible plot could be that James and Lily have been friends for a long time, but there's always been an underlying tension. One day, after an emotional argument, James can't hold back his feelings anymore and kisses her against the wall.
There could be. A lot of fanfiction explores different romantic or intense moments between characters. 'James kisses Lily against the wall' is a very specific and likely scenario that some fans may have written about. Check the relevant fandom sections on major fanfic sites.
One possible theme could be redemption. Maybe James realizes the wrongs he did to Snape and the kiss is a sign of his attempt to make amends. Another theme could be a change in their relationship dynamics, from enemies to something more complex. For example, a sudden realization of a deeper connection.
One possible plot could be that the portraits of James and Lily are somehow damaged or fading, and Harry has to find a way to restore them. He goes on a quest to find ancient spells or magical artifacts that can help.
One challenge could be financial. They might need to figure out how to afford things like baby clothes, a crib, and medical expenses. Another might be emotional stress. Lily could have mood swings during pregnancy, and James might struggle to keep up with her changing emotions.
One possible plot could be that Leni and Lincoln are in a situation where they accidentally kiss during a game or a prank gone wrong. Then they have to deal with the awkwardness and figure out their feelings towards each other.
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.
Well, a plotline might involve them facing challenges from the wizarding world. For example, there could be threats from dark wizards which they have to fight together as a married couple. Or perhaps they get involved in some sort of magical research as a team in their married life. Also, their relationship growth is a big part. They could have little arguments and then make up in the most romantic ways, like James bringing Lily her favorite flowers after a fight.
Perhaps they are blackmailed. A powerful dark wizard could threaten to harm all of their friends and family if they don't abandon Harry. They may feel they have no choice but to sacrifice their relationship with their son to protect others they care about.
In this fanfiction, for the first impression, Lily could be seen as very sweet. James' parents may like her gentle demeanor. However, they might also be a bit concerned that she seems a bit delicate and not strong - willed enough. Or perhaps Lily appears very intelligent from the start, and James' parents are impressed by her knowledge and the way she expresses herself.
It could be an exploration of the darker side of human nature. Writers may use these characters to show how relationships can go horribly wrong, even if it's not in line with the original story.