I think Steve Rogers would be a likely ally. Steve is all about justice and doing what's right, and Castiel has a similar sense of duty in his own way. They could bond over their strong moral compasses.
Bruce Banner could also be an ally. Bruce is a scientist, and Castiel has a vast amount of knowledge about the universe and different forms of existence. Bruce might be interested in learning from Castiel, and Castiel could see Bruce's intelligence as a valuable asset. They could work together to find solutions to problems that require both scientific and otherworldly knowledge.
Iron Man. They both are very intelligent and rely on their technology a great deal. They could share ideas about improving their gadgets and suits.
Black Widow. She has a lot in common with Batman in terms of skills. Both are highly trained in combat, stealth, and espionage. They would understand each other's need for secrecy and their ability to operate in the shadows.
In this fanfiction, Natasha Romanoff could be a great ally for Annabeth. Natasha is a skilled spy and fighter, and Annabeth has her own set of combat and survival skills. They could bond over their shared ability to assess situations quickly and make smart decisions in the heat of battle. Also, Natasha might be interested in Annabeth's knowledge of different cultures and mythologies which could potentially help in their missions.
I think Captain America would be a likely ally. Captain America is all about justice and doing what's right, which aligns well with Gohan's values. They both have a strong moral compass and would respect each other's fighting abilities.
Black Widow would be a likely ally. They both have a background in street - level combat and stealth. They could work together on a mission that requires infiltration and taking down enemies quietly. Their skills would complement each other well, with Daredevil's enhanced senses helping to detect hidden threats and Black Widow's combat training and espionage experience being crucial in executing the mission.
Captain America could be a strong ally for Nightwing. Captain America is a great leader and has a strong sense of justice. Nightwing, coming from Gotham, also has a deep - rooted sense of right and wrong. They could lead the Avengers together in a battle, with Captain America providing the strategic leadership and Nightwing using his agility and combat skills to execute the plans on the ground.
In a 'Castiel meets the Avengers' fanfiction, there could be a lot of interesting scenarios. Maybe Castiel tries to understand the Avengers' concept of heroism which is very different from his own in the Supernatural universe. He might be intrigued by their technology and try to figure out how it works in relation to the magic he is used to.
Thor might be a great ally for Jack Frost. Thor is used to dealing with magic and powerful beings from different realms. He would be less likely to be intimidated by Jack Frost's powers. They could both use their elemental - related powers (Thor with lightning and Jack Frost with ice) to create powerful combined attacks against their enemies. For example, Thor could strike his lightning into Jack Frost's ice structures, creating a powerful blast of energy.
I think Red Hood and Hawkeye could be likely allies. They both have a certain level of street - smartness. Hawkeye is used to working in the shadows and using his skills without relying too much on superpowers, just like Red Hood.
I think Steve Rogers and Will would get along well. Steve is all about honor and doing what's right, and Will has a strong moral compass as a healer. They would probably respect each other's values.