Probably the most difficult part would be dealing with the unrealistic expectations. Novels can create an idealized version of love, and in real life, it's hard to meet those grand ideals all the time. For example, the perfect communication or the unwavering loyalty shown in novels may not be so easily achievable in reality.
Probably the different settings and cultures. Harry Potter is set in a world that is somewhat similar to our own but with a hidden magical society. Star Wars, where the Sith are from, is in a completely different galaxy. The values, social structures, and even the technology are so different. For instance, the concept of light and dark in the two universes is not exactly the same. In Harry Potter, dark magic is clearly bad, but in Star Wars, the dark side of the Force has its own allure and complexity. Trying to make these fit together in a fanfiction would be a huge headache.
Surviving in the ghoul world as a male OC reborn into Tokyo Ghoul fanfiction would be tough due to the lack of a support system. In this cruel world, having someone to rely on can mean the difference between life and death. Without any pre - existing relationships or allies, the OC would have to build them from scratch. This would involve trusting strangers in a world where trust is a luxury. He would also have to learn the unspoken rules and politics among ghouls very quickly to avoid making enemies unknowingly.
The most romantic aspect could be the unspoken understanding between Tobirama and his love. Given his reserved nature, having someone who could understand his thoughts and feelings without him having to say much would be very romantic.
It could be that he takes her to his secret training spots, places that are very special to him. There, they can be alone and he can share with her the real passion he has for his fighting, and she can understand him on a deeper level. This sharing of his most private world is very romantic.
The most romantic aspect could be his unwavering protection. Given his vampire abilities, he can shield his love from any harm. Whether it's from other supernatural threats or just the everyday dangers of the world. His protection is a sign of his deep love and commitment.
I think 'The Brothers Karamazov' by Fyodor Dostoevsky is very challenging. The philosophical depth and intense psychological exploration make it a tough read.
Perhaps the most interesting aspect could be their courtship rituals. Squirrels are known for their energetic and acrobatic displays. The male might chase the female through the trees, leaping from branch to branch in a display of his agility and strength, which is his way of showing her he is a worthy mate.
The most disturbing aspect would be the glorification of two extremely evil individuals. Dahmer and Bundy were serial killers who caused so much pain and death. A 'love story' between them would seem to make light of their crimes and the trauma of their victims, which is completely unacceptable.
The most memorable aspect is the tragic end. Jack and Rose's love was so strong but couldn't overcome the disaster of the ship sinking. It shows the power of love in the face of death.
Well, perhaps it's the plot twists related to this man. In many novels, the story around a character like this can have unexpected turns that keep readers hooked. For example, his relationships with other characters and how they develop in unusual ways.