It could be about the young Avengers reuniting to face a new threat that is related to their past enemies. Maybe a remnant of Hydra has found a way to use old technology against them.
Perhaps in the fanfiction, the Avengers reunite for a more personal reason. One of their own, say Tony Stark (even if he's no longer alive in the main timeline), has somehow sent a distress signal from an alternate reality. They come together, not just to rescue him but also to explore this new and strange reality where things are different. They might meet alternate versions of themselves and have to team up with these versions to solve a problem that is affecting both realities.
Their relationship could become stronger. After being apart and facing the possibility of losing each other, they might cherish each other more. They could start to be more open with their feelings and communicate better.
Captain America would be very important. He has always been the moral compass of the Avengers. His leadership skills and ability to bring the team together would be crucial in a reunion. He can bridge the gaps between different members, whether it's Iron Man's tech - based thinking or Thor's more otherworldly approach.
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.
Maybe Tony has been lost in space or some other difficult situation. When he finally makes his way back, Pepper is waiting at the Avengers compound. Their reunion is filled with emotions. Pepper might be a bit angry at first for him putting himself in danger, but then she realizes how much she missed him and they share a passionate kiss.
It could be a very emotional reunion. Thor might be initially angry at Loki for all his past misdeeds but then quickly soften as he realizes how much he has missed his brother. Loki, on the other hand, might be a bit guarded at first, still not fully trusting Thor's reaction. But eventually, they would embrace and start to rebuild their relationship.
Well, in the fanfiction, their reunion could lead to them teaming up. Maybe they decide to take on a powerful enemy together. They'd combine their skills, with Ace's fire powers and Sabo's fighting abilities, and create a formidable force. They'd also probably have some deep conversations about their past and how much they missed each other.
Their relationship could develop romantically. After the reunion, they might start to realize their feelings for each other more clearly. For example, they could share more private moments, like watching the sunset together while their Pokémon play nearby.
Well, in a fanfiction, their reunion could be in a completely new region. Ash has been invited to a special Pokémon event there and Serena, having heard about it, travels all the way just to see him again. When they reunite, Serena has become an even more confident and skilled Pokémon coordinator. Ash is so proud of her growth, and they decide to team up for a new adventure in this unfamiliar region.