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.
It could be Captain America. He has a strong sense of justice and kindness. So, if he found a dog, he would surely take it under his wing and protect it. Maybe he'd even train the dog to be a mascot for the Avengers.
Thor would be the most likely. Since Thor has a connection to Norse mythology already, he would be familiar with Sleipnir. He might see Sleipnir as a symbol of his home realm and would quickly bond with it. They could communicate in a way that the other Avengers can't, sharing stories of Asgard and the great battles there.
One common plot could be the Avenger taking on a protective role over the baby. For example, they might keep the baby safe from villains who are after it for some special power the baby has. Maybe the baby is related to some important figure in the Avenger's world. Another plot could be the Avenger trying to find the baby's family while forming a bond with the baby during the search. They could face many challenges along the way, like dealing with other characters who misunderstand their intentions with the baby.
Captain America is very likely to be featured. He has that paternal, moral - compass kind of vibe. He would be the one to gently tickle the 'daughter' as a way of showing his soft side. Thor could also be a popular choice. His big - hearted and sometimes child - like nature would make for some great tickle - related antics with the 'daughter' figure.
Iron Man could also be a likely candidate. Tony Stark is known for his charm and his ability to work with all kinds of people. He might be intrigued by Flash Thompson's bravado and see it as a starting point to shape him into a better person. He could offer Flash some cool tech gadgets and in the process, form a friendship. Stark would likely use his wit and intelligence to get through to Flash and show him a different way of looking at things.
Thor would be one of the active ones. He has a lot to share from his Asgardian experiences. Hulk, well, he might not type much but when he does, it would be in short, powerful statements. And Scarlet Witch could be in there too. She could use her powers to sense if there's any danger approaching the chatroom or the Avengers themselves while chatting with the others.
I think Loki would be really interesting as a teenager. He's already a bit of a mischievous character, and as a teen, he would probably be even more so. He might play pranks on the other Avengers - turned - teens and cause all sorts of chaos in their new high - school - like setting.
In addition to the well - known Avengers, there might be some new or less - known characters. Maybe some characters from Iron Man's rogues' gallery could show up. Or perhaps a new hero that is related to Iron Man's technology in some way. There could also be some side characters that are important for the development of the story, like Pepper Potts who has a close relationship with Iron Man.
I think it would be Spider - Man. He's closer to her age in a way compared to the other Avengers. He can understand her more easily, like the things she likes, the new trends, etc. And he has that friendly neighborhood hero vibe that would make the girl feel comfortable around him quickly.
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.