Thor is often considered one of the most powerful Avengers. His godly powers, like his hammer Mjolnir and his ability to control lightning, give him a significant edge.
Loki is often a popular character in Avenger OC fanfic. Many writers create OCs that interact with Loki in various ways, whether it's as an ally, a love interest, or an enemy. His complex personality and backstory provide a lot of material for interesting stories.
I think Thor would be the most interesting. If he turned into a large and powerful animal like a dragon, he could breathe fire and fly. His already - impressive strength and combat skills would be enhanced, and he could be a real force to be reckoned with. He could also use his dragon form to intimidate enemies and protect Asgard or Earth in a very different way.
Captain America is often very popular. His strong moral code and leadership qualities make him a great character to write about in fanfiction. In the context of both Avengers and Agents of Shield, he can be shown leading joint missions or mentoring new recruits. Another popular one is Black Widow. Her mysterious past and combat skills give a lot of room for creative storytelling, like exploring her time before she joined the Avengers or her interactions with Agents of Shield.
In some fanfictions, the 7th Avenger could be a time - traveler from the future. This person has come back to prevent a major disaster that the Avengers couldn't handle without their help. They know all about the future events and use this knowledge to guide the team, but also have to be careful not to change the timeline too much.
Captain America is a likely candidate. He has a strong moral compass and a sense of protectiveness. He would see it as his duty to take care of the baby. His old - fashioned values would make him a great caregiver, like making sure the baby has a proper upbringing even in the middle of all the chaos in the Avenger's world.
Captain America. He values honor, courage, and justice, which are all qualities that Robin also embodies. Cap would respect Robin's skills in combat and detective work. They could lead a team together, with Cap providing the strategic leadership on a large - scale battlefront, while Robin uses his stealth and intelligence to gather intelligence and take on enemies from the shadows.
Hulk would be the most hilarious. Imagine a huge, green cat with a short temper. He would probably be constantly knocking things over with his big cat paws and getting frustrated when he can't smash things as easily as he could in his normal form. His attempts to roar would come out as a strange meow - growl combination that would be both intimidating and comical at the same time.