One idea could be Master Chief getting transported to the DC universe and teaming up with Batman. They could combine their combat skills and strategic thinking to take on a common enemy like Darkseid.
First, you need to decide on the main characters from both universes that you want to feature. For example, if you choose Halo's Arbiter and DC's Wonder Woman, think about how their personalities and abilities will interact. Then, come up with a plot. It could be something like a threat that endangers both the Halo and DC universes, forcing them to work together.
A cool Halo and DC crossover fanfiction could involve Cortana interacting with Oracle from DC. Cortana's vast knowledge and Oracle's skills in gathering information could combine to solve a mystery that spans both universes. They might be trying to figure out why certain artifacts from Halo are showing up in the DC world and what it means for the balance of both universes.
You can also integrate them through shared technology or concepts. For instance, the power source in the Halo universe could be related to some meta - human power in the DC universe. This could lead to a scientific exploration in the fanfiction where characters from both universes collaborate to understand and utilize this connection.
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.
Master Chief is often a popular character. Given his significance in the Halo universe, it's natural for him to be a central figure in crossovers. He has the skills and determination to face the Flood threat. Another one could be the Arbiter. His unique perspective as a former Covenant and now an ally in the fight against common enemies makes him an interesting addition in fanfictions involving the Flood.
The creation of a new threat or goal that unites the characters from different universes is important too. In a Halo Reach and Destiny crossover, a powerful new alien race could be the common enemy that forces the Guardians and Spartans to work together. This gives a purpose to the crossover and drives the story forward as they have to combine their skills and resources to face this threat.
One possible plot could be that the Master Chief is somehow transported to the Avengers' universe. He teams up with Iron Man to stop a new threat that combines Covenant and Hydra technology. Their combined skills in combat and technology would make for an exciting adventure.
One interesting concept could be the crossover of Halo 5 characters with characters from other science - fiction universes. For example, seeing Master Chief interact with characters from Star Wars. It could lead to epic battles or unexpected alliances. Another concept might be the combination of Halo 5's technology with that of a different game world. This could create unique weapon combinations or new ways of traversing the game environment. Also, crossovers can explore different cultures within the Halo 5 universe and how they would react to an outside influence.
One common theme is the clash of different technologies. In these fanfictions, you often see Destiny's space - age, almost magical technology pitted against Halo's more militaristic and hard - sci - fi tech.