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.
Another interesting pairing could be Batman and Daisy Johnson. Daisy has her inhuman powers, especially her seismic abilities. Batman could be intrigued by her powers and try to find ways to use them effectively in combat. And Daisy could learn a lot from Batman's combat training and his no - nonsense attitude towards fighting crime. They could form a strong duo when it comes to facing enemies that require both brute force and strategic thinking.
One really interesting character pairing could be Fitz - Simmons and the TMNT as a whole. Fitz - Simmons are brilliant scientists, and the turtles are a scientific anomaly. They could study the turtles and their mutagenic origin. The turtles could also benefit from Fitz - Simmons' knowledge of advanced technology. Another pairing that would be cool is Agent Ward (assuming he's not evil in this fanfiction) and Leonardo. Ward is a skilled agent, and Leonardo is a skilled ninja. They could exchange fighting and survival skills, and their different perspectives on loyalty and duty could lead to some interesting story arcs.
Definitely Coulson and Iron Man. Coulson is a big fan of Iron Man in the show, and seeing them interact would be great. Iron Man's sarcasm and Coulson's more straight - laced nature would create some really interesting banter.
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.
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 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.
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.
Definitely the Flash and Iron Man. Tony Stark's high - tech gadgets and intelligence combined with the Flash's super - speed would make for some really cool scenes. They could work together on building some sort of super - fast vehicle or weapon. Another interesting pairing would be the Flash and Captain America. Steve Rogers' leadership and combat skills along with the Flash's speed could be a great combination for taking on enemies. The Flash could quickly get Steve into strategic positions during battles.
Inuyasha and Thor would be an interesting pairing. They both have a warrior - like nature. Thor with his hammer and Inuyasha with his sword could have some epic battles together against common enemies. They might also exchange stories about their different worlds and the powers they possess.
Definitely Captain America and Ikaris. Captain America represents the best of human values and leadership. Ikaris has his own strong sense of duty among the Eternals. They could bond over their similar leadership qualities and also learn from each other's different combat and strategic approaches. And then there's Hulk and Thena. Thena's combat skills and Hulk's brute strength could be a very volatile but also exciting combination when they face a tough enemy.