Cersei could be the most interesting in a sense. If she were reincarnated, she could either double down on her power - hungry ways or completely change. Maybe she could learn from her past mistakes and become a more benevolent ruler. Or she could use her reincarnation as a way to exact revenge on those who opposed her in her previous life in more devious and calculated ways.
One idea could be that the reincarnated Lannister has all the memories of their past life. They could use this knowledge to avoid the mistakes their previous self made. For example, not getting involved in certain political schemes that led to their downfall. Another plot could be that they try to change the fate of their family members. Maybe prevent Jaime from losing his hand or stop Cersei from going down her tyrannical path.
Definitely Jaime Lannister. His character had a lot of development in the original series. If reincarnated, he could either redeem himself even further or take a completely different path. He might try to make amends for his past actions by becoming a true knight, or he could turn against his family if he realizes the toxicity of their ways. His reincarnation would bring a whole new set of possibilities to the story.
Reincarnating as Tyrion Lannister would be really interesting. He's already a complex character with a sharp mind. As someone reincarnated, you could use your modern ideas to enhance his political acumen even more. You could also try to change his relationship with his family for the better, or at least make his life less miserable while still being the clever dwarf everyone fears and respects in the political arena.
Jon Snow is quite popular. People like to be reincarnated as him because he has a lot of potential for growth and a mysterious backstory. They can use their new knowledge to perhaps avoid some of his tragedies and rise to power more quickly.
I think being reincarnated as Loki would be super interesting. He's such a complex character with his own motives and schemes. You could take over Loki's life at a certain point and use your modern - day knowledge to either further his plans in a different way or completely change his path. Maybe you could prevent him from getting into so much trouble with Thor and the Avengers, or use his shapeshifting abilities in ways that no one in the Marvel universe has ever seen before.
There are no appropriate'sex stories' in a positive sense about Cersei Lannister. Her character is more about her ambition and her role in the power struggles of the Seven Kingdoms. Her relationships, like with Jaime which was incestuous, were part of her downfall and her complex character arc. It was a way to show the moral decay in the world of 'Game of Thrones' rather than just a titillating story.
Consider their upbringing. If they were raised away from the main Lannister stronghold, they might have a different set of values. Maybe they were raised by a kind - hearted servant who taught them to be more empathetic. So their personality could be a mix of Lannister pride and this new - found empathy. They could be conflicted between loyalty to their family and doing what is right according to their own conscience.
If Naruto was a Lannister in a Game of Thrones fanfiction, he'd be a very different kind of Lannister. He'd probably use his ninja skills to protect the family in ways they've never seen before. Instead of the usual backstabbing, he might bring in a sense of loyalty and honor that the Lannisters lack. He could use his jutsu to make the Lannister stronghold more secure, like creating barriers with his earth - style jutsu.
One possible plot could be a young Lannister male OC who, unlike his power - hungry relatives, is more interested in the arts and knowledge. He could secretly befriend a maester and learn about ancient Valyrian lore. This would put him at odds with his family as they see his pursuits as a waste of time. He might then use his knowledge to try and broker peace during the war, facing many challenges along the way.
I think Daenerys Targaryen would be really interesting. If she were reincarnated, she could use her past experiences to be an even more powerful and just ruler. She already had a complex journey in the original series, and reincarnation would add more depth to her character. She might be able to avoid some of the mistakes she made in her first life, like trusting the wrong people.