Often, the Boltons are main antagonists. Ramsay Bolton in particular is a cruel and sadistic character who has caused great harm to both Theon and Sansa. His actions are a major source of conflict in many fanfictions.
It could also be Littlefinger's followers or remnants of his schemes. Since Littlefinger had his own plans involving Sansa, his influence might still linger in fanfiction, and those who were loyal to him could oppose Sansa and Theon's attempts to bring stability to the North.
One possible fanfic could be where Theon, after surviving, teams up with Jon Snow. They use their combined knowledge of the North and the Iron Islands to fight against the White Walkers. Theon's survival gives him a new perspective, and he becomes a key figure in the battle, using his skills in seafaring to bring in supplies and reinforcements.
Another scenario could be a more romantic one. Theon, having redeemed himself, realizes his true feelings for Sansa. They might have a moment where they share their past traumas and find solace in each other, slowly building a relationship based on mutual understanding and respect.
One interesting storyline could be Sansa and Theon teaming up to take revenge on their common enemies. They've both been through so much in the original series, and in fanfiction, they might use their combined knowledge and skills to outwit those who wronged them.
Theon's survival in a fanfic could lead to him being a key player in preventing certain wars. In the show, many wars and battles happened regardless of his presence. But in a fanfic, his new - found wisdom from his survival experiences could be used to broker peace deals between different houses, something that was not really explored in the show's version of his story.
One great 'Lord of the Rings and Game of Thrones Fanfiction' is 'The Convergence of Westeros and Middle - earth'. It combines the magic systems of both in an interesting way, like how the One Ring could interact with the magic in Westeros.
I really like the fanfic 'A Song of Rings'. In this story, the One Ring somehow finds its way to Westeros. The author does an amazing job of integrating the different lore. For example, how the power of the ring affects the power - hungry characters in 'Game of Thrones' is fascinating. It also shows how the characters from Middle - earth would react to the more brutal world of Westeros.
The 'forbidden love' trope can be really engaging. Imagine a love story between a character from 'Game of Thrones' and one from 'Lord of the Rings' in a crossover fanfiction. This trope can create a lot of conflict and emotional depth. And the 'rag - to - riches' or 'rise to power' trope is also seen a lot, as it shows the character's growth and transformation.