A possible plotline is that Thanos hears about the powerful chakra in the Naruto world and decides to conquer it. Naruto stands up to him, and during the fight, Naruto unlocks a new form of his power that is on par with Thanos' strength with the Infinity Stones. Along the way, Naruto teams up with some of his old foes who also realize the danger Thanos poses. They all fight together to protect their world.
One possible storyline could be that Thanos arrives on Earth seeking a powerful artifact. Superman steps in to stop him. They battle it out across the city, with Superman using his strength and speed to try and take down Thanos. Another could be that Thanos somehow manages to get Superman under his control using the Infinity Stones, and then the rest of the Justice League has to figure out how to free Superman and defeat Thanos.
In some fanfictions, there could be a situation where Thanos is actually related to an ancient dark magic in the Harry Potter universe. Harry has to figure out this connection and use it to his advantage. Maybe Thanos is after a powerful magical artifact in the wizarding world that he believes will enhance the power of the Infinity Stones. Harry and his friends then have to protect this artifact while also trying to defeat Thanos, which could lead to a series of epic battles and magical duels.
In a Loki x Thanos mpreg fanfiction, a potential plotline could be that the pregnancy is part of a prophecy. Loki and Thanos have to deal with the implications of this prophecy, whether it means the child will bring great power or destruction. There could be a lot of internal struggle for Loki as he contemplates his role as a soon - to - be parent, especially considering Thanos' nature. And Thanos could be using the situation to gain an advantage over his enemies, all while trying to protect Loki and the unborn from those who would oppose them.
One possible plotline could be Thanos trying to win Tony over while still being true to his villainous ways. Another could be Tony using Thanos' feelings to his advantage in a battle.
Another storyline could be that Thanos, aware of One Punch Man's power, tries to use the Infinity Stones to either eliminate him or make him an ally. There could be a long - drawn - out battle where One Punch Man has to constantly dodge Thanos' attacks while trying to find a way to land his one punch. In the end, he manages to do it and saves the day.
A plotline could involve Kudo Shinichi being brought to Gotham, either by accident or some sort of experiment. There, he starts solving crimes in his own way, which catches Batman's attention. Batman might initially be suspicious of this new 'detective' in town and they could end up in a rivalry until they realize they can work together to take down a greater evil like the Joker who has some new, devious plan that affects both their worlds.
One possible plotline could be that Star discovers belly dance while exploring a new planet and decides to incorporate it into her magical practices. It could add a new dimension to her powers.
Well, in a fanfiction, a plotline could be that Percy Jackson is on a quest and somehow crosses paths with Voldemort. Voldemort senses Percy's power and wants to study it. Percy, not knowing much about the wizarding world, initially mistakes Voldemort for just another monster. As they interact more, they realize that their worlds are more intertwined than they thought. Another plot could involve a powerful artifact that both of them are after. The artifact has the power to change the balance of both their worlds. So they are in a race against each other to get it, but along the way, they start to understand each other's motives and might even consider working together to keep the artifact out of the wrong hands.
One possible plotline is that Ash is on a journey to prove himself against the toughest opponents and finally faces Brandon. He uses his Pikachu in a unique way to gain an edge in the battle.
Maybe Harrison comes to Ash's town and challenges him. Ash accepts, and during the battle, there are flashbacks of their past experiences and how they got to where they are as trainers. This could add depth to the story and show their growth.