One idea could be exploring their life after the war. For example, how they adjust to a more peaceful existence. Maybe they start a school in Essos to teach children about different cultures. Another plot could involve them going on a journey to find Missandei's long - lost family. They could face various challenges on the way, like dealing with bandits or surviving in harsh terrains.
A possible plot in the fanfiction could be that Grey Worm and Missandei get involved in a political intrigue in Meereen. There are factions that want to undermine the new order they are trying to build. They have to use their wits and the loyalty of their friends to stay in power and keep the city prosperous. This would show their growth from just being soldiers to political leaders.
Perhaps a plot where Grey Worm and Missandei are faced with a magical threat. A dark sorcerer emerges who wants to bring chaos to the lands they are protecting. They have to seek help from other characters in the Game of Thrones universe, like a maester who knows about ancient magic. They then learn about their own inner strengths and the power of love to defeat the sorcerer.
In the fanfiction, their relationship could develop more slowly. Instead of quickly falling in love, they could start as comrades with mutual respect. They might have misunderstandings at first due to their different upbringings. Grey Worm as a former slave and trained killer, and Missandei as an educated interpreter. As they work together more, they gradually realize their feelings for each other.
Another fanfiction could focus on an alternate ending for Missandei. Instead of her tragic death in the show, she could have been the key to a peace treaty between different kingdoms. She uses her diplomatic skills and understanding of various cultures to broker a deal that no one else could.
One idea could be exploring what would happen if a minor character like Gendry had been secretly trained in magic. He could use his newfound powers to unite the smallfolk against the warring nobles. Another plot could revolve around a female knight from the North who disguises herself as a man to fight in the wars, and along the way, she discovers secrets about the White Walkers that no one else knows. Or perhaps a fanfiction where Daenerys and Sansa form an alliance much earlier and together they reshape the power dynamics of Westeros.
A really cool 'Game of Thrones fanfiction' plot could involve the resurrection of some of the dead characters in a different way. For example, what if Ned Stark was brought back not as a vengeful wraith but as a wise advisor to a new king or queen? His honor and wisdom could shape the future of Westeros in a completely new direction. And it would be interesting to see how the other characters react to his return.
Perhaps a fanfiction where Daenerys Targaryen lands in Westeros earlier and forms an alliance with the Starks before the events of the main series. This would completely shift the political landscape and the fight for the Iron Throne. She could bring her dragons to help fight the White Walkers from the start.
One idea could be exploring what would happen if a minor character, like Gendry, had a more central role in the fight against the White Walkers. He could lead a group of common folk and prove that the power doesn't always lie with the nobles.
Missandei played a role in the power politics as well. Her influence, though not as overt as some of the other characters, was felt in the decisions made in Daenerys's court. She was a voice of reason and often tried to balance out the more extreme views. Her character added a layer of complexity to the story, especially when it came to the interactions between different regions and their respective representatives.
One interesting plot idea could be the return of a long - thought - dead character with a new mission. For example, if Ned Stark somehow came back and had to navigate the even more complex political landscape of Westeros. Another could be the exploration of the relationship between Daenerys and Jon Snow in a more in - depth way, perhaps with some new challenges that test their alliance.
Another plot could involve a goddess who was once worshipped in ancient Westeros but forgotten over time. She resurfaces during the chaos of the Game of Thrones. This goddess has the power to manipulate the elements and she decides to test the worthiness of the claimants to the Iron Throne by sending various trials their way.
Perhaps a plot where the gamer character discovers a hidden aspect of the magic in the Game of Thrones world through their unique 'gamer' perspective. This could lead to new powers being unlocked and changing the power dynamics among the different houses.