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.
The plot might center around a new super - weapon. In the Halo universe, there are powerful weapons like the Halo rings. Imagine if some evil force in the Marvel universe gets hold of this technology. The Avengers and Halo characters would have to work together to stop it. For example, Captain America's leadership skills and the Chief's military tactics would be crucial in formulating a plan to retrieve or destroy the weapon.
I think Master Chief and Captain America would be an interesting pairing. Master Chief is a super - soldier in his own right from a high - tech future, while Captain America is a super - soldier from the past. Their different combat styles and values would create an interesting dynamic.
Thor and the Arbiter would be a great pair. Thor is a god with incredible strength and powers, and the Arbiter is a respected warrior in the Halo universe. They could exchange stories about their different cultures and fight side - side. Thor's lightning powers combined with the Arbiter's energy sword and combat skills would be a force to be reckoned with against any enemy.
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.
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 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.