One idea could be a plot where the Avengers and DC heroes are brought together by a powerful cosmic entity that threatens both universes. They have to combine their powers and strategies to defeat it. For example, Superman and Thor could work together to take on the entity's brute strength while Batman and Iron Man figure out its weaknesses using their intellect.
I think Thor and Wonder Woman would make an amazing pair. They both come from god - like lineages. Thor with his Asgardian powers and Wonder Woman with her Amazonian and divine powers could have some epic battles together against common foes. Their different yet powerful fighting styles would complement each other well.
Another idea is that Greninja gets exposed to some kind of meta - human - creating substance in the DC universe and gains new powers. It could then join the Teen Titans, sharing its unique Pokémon abilities with the group. It would be interesting to see how Greninja interacts with characters like Robin, Starfire, and Cyborg, and how it uses its powers in battles against the Titans' enemies.
They might be quite confused at first. Since the DC and Marvel universes have different settings, powers, and character personalities. For example, Superman's powers are very different from those of any Avenger. They may also find some interesting ideas in the fanfiction, like unique crossovers that DC fans imagine between their characters and situations that could be completely different from what they're used to in the Marvel world.
One idea could be that Godzilla is drawn to the energy source of one of the Avengers' tech, like Stark's Arc Reactor. This causes chaos as Godzilla approaches the city where the Avengers are based. The Avengers initially see Godzilla as a threat and try to fight it. But then they realize that an even bigger alien force is using Godzilla's presence as a distraction. So, they have to find a way to communicate with Godzilla and work together to defeat the common enemy.
One idea could be that Nightwing is transported to the Avengers' universe through some sort of interdimensional rift. He then has to team up with the Avengers to fight a common enemy, like Thanos who has somehow acquired some of Batman's tech. Nightwing's acrobatic skills and detective abilities would be a great addition to the Avengers' power set.
Maybe there's a multiverse event that causes Peter Parker to be transported to the DC universe. He meets Superman first, and Superman is initially a bit wary of this strange new hero with spider - powers. But as they face a common enemy, like Lex Luthor who has somehow gotten his hands on some Oscorp tech, they learn to work together. Peter can use his scientific knowledge from his own world to help Superman figure out how to disable the dangerous tech.
Well, first, a good DC crossover fanfiction has to have interesting character interactions. For example, seeing how Batman would react to meeting Wonder Woman for the first time in a new context. Second, a unique plot. Instead of the typical 'save the world' plot, it could be something like the heroes having to solve a mystery that involves different dimensions. Third, accurate portrayals of the characters. If Superman is in it, he should still be the big - hearted, powerful hero we know.
Definitely Naruto and Superman. They are both iconic characters in their respective universes. Their different power sets make for an interesting comparison. Another popular crossover is between Sasuke and Batman. They both have a dark and brooding side. And of course, Sakura and Wonder Woman. The contrast between Sakura's ninja skills and Wonder Woman's superhuman abilities is appealing.
A really cool concept could be a story where the universes merge and the Avengers and DC characters have to adapt to a new reality. They might have to deal with new power dynamics and form new alliances or rivalries.