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.
In this fanfiction, Skynet could find a way to connect to the Matrix. It might try to take over the Matrix and use it to control more humans. The machines in the Matrix might initially resist, but Skynet could offer them a deal to share power over humanity.
Another possibility is that a Marvel hero, say Spider - Man, accidentally ends up in Beach City. He teams up with Steven to deal with a local threat that is also related to some sort of interdimensional chaos. For example, a group of villains from the Gem world and the Marvel world are working together to open portals to other dimensions to steal valuable resources. Steven and Spider - Man would have to use their unique abilities to stop them.
To write this kind of fanfic, start with a strong hook. Maybe start with Steven waking up in the Underground or Frisk arriving on Earth near Beach City. Then, build the story gradually. Show how the characters react to the new environment and each other. Don't forget to add some action and conflict. It could be a battle between the Gems and some of the more aggressive monsters from Undertale. And make sure to stay true to the essence of both universes. Don't change the core personalities of the characters too much. This way, fans of both shows will be able to enjoy your fanfic.
Well, the 'team - up' trope is quite popular. Imagine Batman and Sherlock Holmes working together to solve a mystery that spans across their respective universes. Then there's the 'love - across - universes' trope. Characters from different universes fall in love, which creates a lot of interesting drama and conflict, especially when they have to deal with the differences in their worlds. And the 'lost - in - a - new - universe' trope where a character has to find their way back home while navigating the strange new rules and inhabitants of the crossover universe.
If there was a crossover with the Spider - Man universe. Peter Parker would be in awe of Wanda's powers. He would probably try to ask her a million questions. This could make Vision a bit jealous as he sees Peter as a bit of a pest. Meanwhile, Aunt May would probably invite Wanda and Vision over for dinner, and in that domestic setting, we would see a whole new side of their relationships. Wanda might start to see Vision in a more human way as she observes the relationship between Peter and Aunt May.
Peridot is quite popular too. Her unique personality, especially her growth from a somewhat antagonist to a more friendly character, makes for great storytelling in crossovers. In a crossover with 'Ben 10', Peridot's technological knowledge could be paired with Ben's Omnitrix - related adventures.
First, know your characters well. Since it's a crossover, understand the traits and backstories of characters from different DC universes. For example, if you're crossing over Batman and Aquaman, know how Batman's detective skills and Aquaman's underwater abilities can interact. Second, create a unique plot. Don't just reuse existing storylines. Maybe set the story in a new location or have a new villain that forces the characters to work together in unexpected ways.
One great 'dc universe online crossover fanfiction' is 'The Flash and Green Lantern: Cosmic Runners'. It combines the speed of The Flash with the power of Green Lantern in an exciting adventure across different dimensions. The story has a fast - paced plot and well - written character interactions.