One interesting plotline could be a magical adventure where the two couples, Malec (Magnus and Alec) and Solangelo (Nico and Will), team up to solve a mystery that threatens the shadow world. Maybe they have to find a lost artifact that has the power to disrupt the balance between the mortal and magical realms.
The main characters are from two different pairings. First, there's Magnus Bane, a powerful warlock, and Alec Lightwood, a Shadowhunter from the 'Malec' pairing. Then from the 'Solangelo' pairing, there are Nico di Angelo, a son of Hades, and Will Solace, a son of Apollo.
Often, the plot involves Will misinterpreting Nico's actions or words as signs that he might be losing him. For example, if Nico is being secretive about something, Will might assume the worst and his jealousy flares up. This then leads to some sort of conflict between them that they have to work through, usually with a lot of emotional conversations and realizations about their relationship.
Well, they could team up to fight a common enemy. Maybe the Avengers are facing a magical threat and Solangelo's skills in the magical realm come in handy.
One possible plotline could be Yelena initially being distrustful of the Avengers. Maybe she has been taught to view them as enemies, but as she gets to know them, she realizes they have similar goals. For example, she might find common ground with Natasha's former teammates in their fight against evil.
One interesting plotline could be Luigi and Daisy teaming up to solve a mystery in the Mushroom Kingdom. Maybe there's a strange disappearance of power - ups, and they use their unique skills to figure it out.
One interesting plotline could be Harry somehow traveling back in time to meet Grindelwald in his prime. This could lead to a battle of wits between them, with Harry trying to stop Grindelwald from starting some of his more heinous plans early. Another might be that Grindelwald, seeing Harry's power, tries to recruit him to his side, offering him power and knowledge that Harry has to resist.
One possible plotline could be that Caroline and Hope initially have a misunderstanding when they meet. Maybe Caroline misinterprets Hope's intentions, leading to some tension at first. Then as the story progresses, they discover they have common goals, like protecting their families or the supernatural community they belong to.
One interesting plotline could be their initial meeting being filled with suspicion but gradually turning into mutual respect as they realize they both have a common goal of saving the realm. For example, Jon might be initially wary of Daenerys and her dragons, but as they talk more, they find they can work together against the White Walkers.
In some 'itachi meets sarada fanfiction', Itachi could be brought back to life temporarily. He and Sarada might go on a journey together to face a new threat to the village. During this journey, Itachi could act as a mentor to Sarada, teaching her how to handle difficult situations and how to use her Sharingan more effectively.
Another plotline could involve Petunia getting involved in one of the Marauders' adventures by accident. Maybe she stumbles upon them while they are exploring a secret part of Hogwarts and has to work with them to get out of a tricky situation.