Daenerys could be a likely character. She has a strong will, and seeing her being hypnotized or using hypnosis on others would be an interesting twist. It could play into her power dynamics, like hypnotizing her dragons to perform different tasks.
Cersei is another one. Given her complex personality and her thirst for power, she might either be a victim of hypnosis or use it herself to further her own goals. Hypnotizing her enemies or those in her court to gain more control over the Seven Kingdoms would be a fascinating plot point.
Theon Greyjoy is also a candidate. His identity and self - worth are often tied to his appearance. A headshave in fanfiction could be a way to explore his further degradation or, conversely, his redemption as he sheds his old self - image and tries to start anew.
Melisandre is a likely candidate. She already has some mysterious powers in the original series, so in fanfiction, it's easy to imagine her delving deeper into mind control. She could use it to sway Stannis Baratheon more effectively or even target his enemies.
Daenerys could be in it. She has a strong personality and is used to being in control. If someone challenges her or her claim to the Iron Throne in the fanfiction, it could lead to a catfight. Also, Margaery. She's clever and manipulative, and a catfight with her could be very interesting as she tries to outwit her opponents using her charm and wits.
Theon Greyjoy. He was often in situations where he was bullied and belittled. It's not a stretch to think that some of Ramsay Bolton's men might have given him a wedgie as part of their torture and humiliation tactics.
Arya Stark would also be a good choice. Her assassin skills could be put to use in a modern - day context. Maybe she'd be hunting down criminals instead of her enemies in Westeros. And her independent spirit would make her an interesting character to follow as she adapts to the real world.
Ciri and Daenerys Targaryen. Ciri has magical powers and is a strong female character. Daenerys has her dragons and is a powerful leader. They could meet and form an alliance. Ciri might be interested in Daenerys' dragons and Daenerys could be intrigued by Ciri's magic. Together they could change the power dynamics in a fanfiction world.
Tyrion Lannister. He is known for his wit and political acumen. Meeting Kratos, a powerful and straightforward warrior, would create an interesting dynamic. Tyrion might try to use Kratos for his own political games or just be fascinated by his stories of battles from another world.
The main character is surely Robb Stark in 'robb returns game of thrones fanfic'. Alongside him, his bannermen who were loyal to him before his supposed death would be there. These include men like Greatjon Umber. His enemies from before, like the Boltons, would also be important characters as they would be directly affected by his return. His return might mean vengeance for what they did to him. And his friends and potential allies from other regions, such as some of the Free Folk if the story allows for it, could also be part of the narrative as they might see an opportunity in allying with Robb to change the power balance in Westeros.
Another possibility is Cersei and Jaime. Their relationship is already very complex and twisted. In a fanfiction, it could be a different take on their relationship where Jaime is seen more as a king - like protector to Cersei, and she finds herself in his arms in a more tender and loving way than their usual dynamic in the show.
Homer Simpson is very likely to be involved. He's the main character and his bumbling yet loveable nature can create a lot of comical situations in crossovers. For instance, if it's a crossover with 'The Flintstones', Homer would probably be trying to out - eat Fred Flintstone at the local diner.