Phil Coulson is an obvious choice. He's a fan favorite from Agents of SHIELD. His leadership skills, knowledge of SHIELD operations, and his ability to deal with all kinds of threats could be put to great use in a crossover. He could be the one to bridge the gap between the two groups. The Avengers might initially underestimate him, but as the story progresses, they would come to respect his abilities and his no - nonsense approach to getting things done.
I think Daisy Johnson would be really interesting. She has her unique powers and a complex backstory from her time in SHIELD. In a crossover, she could interact with the likes of Thor. Maybe she could learn some new ways to control her powers from him, as he has experience with his own godly powers. And she could also show the Avengers her skills in combat and stealth that she learned in SHIELD. It would be fascinating to see how she holds her own among the super - powerful Avengers.
Loki could be a great focus in a fanfiction involving both. He has a history with the Avengers, but his wily and unpredictable nature could lead to some interesting interactions with the Agents of Shield. He could be playing his own game, pretending to help both groups while actually having his own agenda. It would be fun to write about how the Agents and the Avengers try to figure out his true motives and keep him in check.
Definitely Captain America and Agent Coulson. Captain America represents the old - school heroism of the Avengers and Coulson is the heart and soul of Agents of Shield. Their different leadership styles and moral compasses would create a great dynamic. They could learn from each other, like Coulson teaching Cap about modern technology and Cap teaching Coulson about the values of the past.
Daisy Johnson from Agents of Shield would be really interesting. Her own Inhuman powers make her a natural bridge between the two groups. She could help the Inhumans understand the ways of Shield and vice versa. Also, Black Bolt from the Inhumans is fascinating. His power is so extreme that just speaking could cause destruction. How would he interact with the Agents of Shield? What kind of strategies would they have to come up with to communicate with him? It would be really engaging to explore.
I think the interaction between Coulson and Captain America would be really interesting. Coulson is a big fan of Captain America, and Steve would be impressed by Coulson's leadership and the way he holds his team together. They could exchange stories about their different experiences in the war - Steve from World War II and Coulson from more modern conflicts.
I think Natasha Romanoff and Melinda May would be an interesting pair. Both are strong, badass women with a lot of combat experience. They could share their stories of their past missions, and also train together. Maybe they could even have a friendly competition to see who is better at hand - to - hand combat or espionage.
Definitely Coulson. He has such a rich backstory and his relationship with both the Agents of Shield and the Avengers (in an indirect way) is very interesting. His resurrection in Agents of Shield also provides a lot of potential for fanfiction. You could explore how he copes with being brought back to life and his new purpose.
One idea could be that the Agents of Shield discover a hidden threat that is related to an old Avengers enemy. For example, a Hydra remnant has found a way to re - activate some of the technology left behind by Ultron. The Agents of Shield then have to team up with the Avengers to stop this new threat. They could combine their skills, like the Agents' espionage and the Avengers' super - powers.
One idea could be that the Agents of Shield discover a new threat that is related to an old enemy of the Avengers. They have to team up to figure out how to stop it. For example, a hidden Hydra cell has found a way to re - activate an old weapon that was once used against the Avengers.
Perhaps a virus is released that targets enhanced individuals. The Avengers are directly affected, but the Agents of Shield has some research on a similar virus from a previous mission. They team up to find a cure. Along the way, they face off against a group of mercenaries who are trying to use the chaos to steal valuable technology from both the Avengers and Agents of Shield.
I think the interaction between Bruce Banner and Fitz - Simmons would be fascinating. Fitz - Simmons are brilliant scientists, and Banner is a scientific genius himself. They could collaborate on some new scientific research, like finding a way to control the Hulk better or developing new energy sources. Also, Clint Barton and Skye could have an interesting dynamic. Clint's no - nonsense approach and Skye's hacker - turned - agent background would make for some great banter and teamwork.