Well, in an 'Aint no grave avengers fanfic', it might be about an Avenger who has been presumed dead but actually isn't. They come back with a vengeance, having faced some sort of otherworldly experience in the 'grave'. Their return shakes up the team dynamics as they have changed during their time away. They might bring back knowledge or powers that are both a boon and a challenge to the rest of the Avengers.
A really interesting plot in this kind of fanfic could involve the concept of resurrection magic or technology. The Avengers discover a way to bring back fallen heroes or villains, but it comes with a great cost. They have to deal with the moral implications of this. Is it right to bring someone back? What if the person comes back wrong? This could create a lot of internal conflict within the team, as well as external threats as the consequences of this resurrection play out.
The relationships could develop in a very intense way. For example, if an Avenger comes back from a near - death experience, their comrades might be more protective of them. Or if a villain returns, it could make some of the Avengers who have grudges against that villain even more determined to take them down, strengthening their resolve and the bond within the team.
A plot could revolve around a self - insert being transported back in time and having to gather the Infinity Stones before Thanos does. Along the way, they form unexpected alliances with the Avengers from different timelines. They might start with finding the Space Stone and use it to travel between planets in search of the others.
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.
One idea could be exploring what if Loki had an unexpected ally within the Avengers. Maybe a character who was initially on the heroes' side but got swayed by Loki's charm or promises.
A more complex plot could involve the Flash's speed force interacting with the Infinity Stones. The Avengers might be trying to figure out how to use or contain the power of the speed force in relation to the Stones. This could lead to all kinds of conflicts and alliances within the story, as different characters have different ideas about how to handle such a powerful and unknown combination of forces.
Maybe a story where the Avengers are transported to Supergirl's universe. They have to adapt to her world's rules and help her fight against a powerful villain there. This could involve a lot of world - building and character development as the Avengers learn about Supergirl's allies and enemies.
A plot could revolve around the Avengers Prime traveling through time to prevent a future disaster that they somehow caused. Along the way, they meet different versions of themselves and have to make tough decisions about changing the timeline.
A great plot could involve a time - travel aspect. Batman discovers a device in the Batcave that sends him back in time to the early days of the Avengers. He has to work with a young and inexperienced Avengers team to stop a threat that, if not stopped, will change the future in a bad way. Maybe this threat is an ancient evil that was sealed away but is now being awakened. Or, there could be a plot where Batman's rogues gallery teams up with some of the Avengers' villains. The combined forces of villains create a huge problem, and Batman and the Avengers have to put aside their differences and strategies to take them down. Also, a plot where an alien race is interested in Batman's technology and kidnaps him. The Avengers rescue him, and in return, Batman helps them upgrade their own tech to fight off the alien race's invasion.
One idea could be an imagine where a new superhero joins the Avengers. They have a unique power that the team initially struggles to understand but eventually learns to utilize effectively. For example, a hero with the power to manipulate time on a small scale, like being able to slow down time for a few seconds during a battle.
A good plot could involve a villain from the Homestuck universe creating chaos in the Avengers' world. The Homestuck characters come to help, but there are misunderstandings at first between them and the Avengers. However, as they start to understand each other's powers and motives, they form a strong alliance to defeat the villain.