Well, without actually reading it, it could be about Peter (presumably Peter Parker, Spider - Man) and his adventures related to the Infinity Stones in an Avengers - centric fanfiction. Maybe he's trying to protect the Stones from villains or help the Avengers use them in some new way.
Peter is surely the central character. In addition to the well - known Avengers, there might be other characters too. For instance, Doctor Strange could be involved as he has knowledge about the Stones. Also, villains who are after the Stones might also be considered main characters in a sense as they drive the plot forward. It's all about how Peter and the Avengers interact with these characters in the context of the Stones.
Well, it could be that the'stones from the riverbed' are related to a character's origin. Perhaps a new Avenger has a special connection to these stones. They could have been found by the character as a child in the riverbed near their home, and these stones hold some kind of memory or power that becomes relevant in the story. It could also be a MacGuffin, something that drives the plot forward without having a specific meaning in itself but is important because everyone in the story wants it.
Well, in many fanfictions, the Avengers protect Peter by always having his back in battles. For example, Captain America might shield him from incoming attacks with his iconic shield. They also offer him guidance. Tony Stark could be constantly giving him advice on how to improve his suit and combat skills.
Tony Stark might use his quick wit and technology. He could immediately scan Peter to make sure there's no physical danger causing the hyperventilation. Then he'd probably crack a joke to try and calm Peter down, like 'Kid, you breathe any faster and you'll fly away like a human jetpack.'
They could be incorporated as a quest item. The Avengers might hear about these special stones from the riverbed and set out on a journey to find them. Maybe they need the stones to stop an impending doom like a massive alien invasion.
Natasha Romanoff could show her emotions in a more subtle way. She might look at Peter's old things and a small smile would cross her face as she remembers his innocence and determination. Thor, on the other hand, might become more determined to protect the world in Peter's honor, as he respected Peter's strength despite his young age.
It might be due to jealousy. Peter is young and has a lot of potential. Some of the Avengers could feel threatened by his rapid growth and popularity, especially if he starts getting more attention from the public or from S.H.I.E.L.D. than they do.
Because Peter is often portrayed as a young and inexperienced hero in fanfiction. He's like a little brother to the Avengers, so they feel a sense of responsibility to protect him.
One common theme is mentorship. Often, characters like Tony Stark take Peter under their wing. Another theme is Peter's growth as a hero, learning to use his powers better. Also, the theme of friendship within the Avengers team is common.