Maybe there are some villains. For example, an enemy general who is trying to stop Harry from achieving his goals with the tank. There might also be some side characters like a mechanic who helps Harry fix the tank when it breaks down in the middle of his journey.
One idea could be a racing adventure where Thomas, with his train friends, decides to have a friendly race against Lightning McQueen. They could set up a track that combines the roads for Lightning and the rails for Thomas. Along the way, they face various challenges like broken tracks and potholes on the road, but they help each other out and learn about the different ways of traveling.
A 'tiger tank crossover fanfic' could be about many things. It might involve the Tiger tank from World War II being crossed over into a different time period, like modern day, where it's used in a fictional military scenario. Or it could be crossed over into a fantasy world, perhaps with magic and mythical creatures, and the Tiger tank becomes a strange and powerful object in that setting.
One possible plot could be that the mermaid is captured by a scientist who wants to study her unique biology. The mermaid is initially scared and tries to escape but as time passes, she starts to form a bond with the scientist who begins to question the ethics of keeping her in the tank.
The story in the 'naruto finds a tank fanfic' could also take a more comical turn. Naruto could accidentally set off some alarm on the tank and then start panicking, thinking it's about to attack him. Or he might mistake the tank for a really big and strange animal and start trying to communicate with it in his own Naruto - like way, which would be really funny to read.
One popular 'naruto tank fanfic' could be 'The Legendary Tank Naruto' which features Naruto with some unique and overpowered tank - like abilities. It has an interesting plot where Naruto uses his newfound powers to face powerful enemies in the Naruto universe.
One interesting story could be about a magically enhanced tank in the Harry Potter universe that helps protect Hogwarts during a major battle. It might have spells cast on it to make it invisible or to shoot magical projectiles.
A simple plot could be that Lightning McQueen gets lost and ends up near the railway where Thomas the Tank Engine operates. Thomas offers to help Lightning find his way back to Radiator Springs. Along the way, they have to cross different terrains and deal with some bad - tempered vehicles. But they manage to work together and in the end, Lightning invites Thomas to visit Radiator Springs someday.
The captor might be initially cold and distant, seeing the mermaid only as a curiosity. But as time goes on, they could become more empathetic. For example, if it's a fisherman who captured the mermaid, he might start to feel guilty when he sees her sad eyes. The mermaid, on the other hand, would be hostile at first, trying to bite or splash the captor. But she might also try to communicate her needs, like asking for fresh water or more space in the tank.