Black Bolt from the Inhumans would be really interesting. His power of destructive voice is so powerful that he has to be extremely cautious. In a fanfic, he could be the key to solving a major problem, but his inability to communicate easily (because of his power) could create a lot of tension and drama. The Avengers would have to learn to work around his unique situation.
Karnak from the Inhumans is another good choice. His ability to find the weakness in anything could be crucial in a story where the Avengers and Inhumans are facing a seemingly invincible enemy. He could be the one who discovers the key to defeating the foe, but he might also struggle with the different fighting styles and mindsets of the Avengers.
Scarlet Witch from the Avengers could be a great focus. Her reality - warping powers could interact in fascinating ways with the Inhumans' abilities. Maybe she accidentally causes some chaos among the Inhumans and then has to work with them to fix it. She could also form an unlikely friendship with an Inhuman who has a similar outcast feeling because of their powers.
One plot idea could be that the Inhumans' city of Attilan is in danger and the Avengers are called in to help. The Inhumans are initially hesitant to accept their aid due to their differences, but they soon realize they need to work together. Another idea is a young Inhuman with a powerful but uncontrollable ability is found by the Avengers. They try to help the Inhuman learn to control it while also protecting them from those who would use their power for evil.
In my opinion, it would be Tony Stark. His relationship with Peter Parker is already quite complex. He is like a mentor to Peter, but at the same time, he has his own issues and motives within the Avengers. In a fanfic, we could explore how Tony's decisions regarding Peter affect the whole Avengers dynamic. He could also use his genius to develop new tech for Peter, which could lead to some exciting storylines. For example, creating a new suit for Peter that has some unique abilities that come in handy during a big battle against a powerful enemy.
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.
Thor could also be a great character to focus on. He has his Asgardian powers and a different cultural background compared to the others. In a fanfiction with Doctor Strange and the Avengers, he could be the one who has the most trouble understanding Doctor Strange's magic at first. His reactions to the strange magical situations and how he tries to incorporate his own strength and honor into the mix could create some really engaging storylines. Also, his relationship with other Avengers and how it changes with the addition of Doctor Strange's presence could be explored.
Captain America's protege would be an interesting choice. He has big shoes to fill, and seeing how he has developed in the absence of Captain America would be engaging. He might struggle with living up to the legacy, but at the same time, he could bring his own unique style and leadership to the group during the reunion. His relationships with the other young Avengers, especially those who knew Captain America well, could be a major part of the story.
I think Spider - Man himself would be the most interesting. He's a young and relatively inexperienced hero compared to the Avengers. His different perspective and his unique powers like climbing walls and the spider - sense would add a lot of flavor to the story. He could also bring in a sense of humor and a bit of naivete that would contrast with the more jaded Avengers.
Captain America would be really interesting. His moral compass and leadership skills in a time - travel scenario would be fascinating. He has to deal with the changes in the past that the time travel causes, and also make sure the other Avengers stay on the right path. He might struggle with seeing his old world again and having to make choices that could change it further.
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.
Vision is definitely an interesting character. His unique origin as a combination of JARVIS, the Mind Stone, and Tony Stark's creation makes for a lot of potential exploration. Wanda Maximoff is also great. Her powers are complex and her backstory of loss and revenge can be delved into deeper. And of course, Ultron himself. His view of humanity and his extreme measures to 'save' the world can be examined from different angles.
I think Jack - Jack would be really interesting. His unpredictable and ever - changing powers could bring a lot of chaos and fun to the story. He could be the wildcard that the Avengers don't quite know how to handle at first. Maybe he accidentally turns into something that causes a big problem, like a giant monster, but then uses his powers in a creative way to help solve it later.