I think another important theme is sacrifice. Itachi sacrificed his reputation, his relationship with his family and his own life in a way. He lived in the shadows, in this 'darkness' of secrecy and lies, all for the 'light' which was the safety and future of the Hidden Leaf Village. His story in the novel is a complex exploration of these ideas of sacrifice, light and darkness.
In Itachi yaoi fanfiction, Itachi is often depicted as a complex and mysterious character. He is usually highly intelligent and powerful, which makes him an alluring figure in romantic or relationship - based stories. His loyalty, whether it be to his family, his village, or his love interest in the yaoi context, is also a common characteristic explored.
Sasuke is often paired with Itachi in these fanfictions. Since they are brothers, when Itachi time - travels, his interactions with Sasuke are crucial. Sasuke might be at a different stage in his life, and Itachi could try to influence his development. Another common character is Naruto. Naruto's ideals of friendship and peace could contrast with Itachi's more complex and tragic view. Their interactions could lead to interesting plot developments.
He could also develop through facing new threats in the RWBY world. The Grimm are different from his usual opponents in Naruto. Dealing with them might force him to adapt his combat style and in turn, grow as a character. For instance, he might have to rely more on others' skills in a team - based RWBY battle scenario.
One reason could be that he sees something special in Sakura. Maybe he admires her strength or her kind heart, and he doesn't want to lose her to someone else who might not appreciate her as much as he does.
His possessiveness could be seen in his actions towards Sakura. He may be overly protective, like not allowing her to go on dangerous missions alone. He would always want to be the one to accompany her, ensuring her safety at all times. This shows that he doesn't want to share her with any potential threats that could come her way during a mission.
Wonder Woman is another character that might be paired with Itachi. Her Amazonian warrior heritage and Itachi's shinobi skills can be compared and contrasted. Their different views on justice, with Wonder Woman coming from a more idealistic place and Itachi having a more pragmatic approach, can lead to some great character - driven storylines in fanfiction.
One interesting aspect of Naruto Rule 63 Itachi fanfiction could be exploring Itachi's character from a different gender perspective. In these fanfictions, Itachi might have different relationships with other characters. For example, his bond with Sasuke could be portrayed in a new light, perhaps with more emotional nuances due to the gender - swap. Also, the way he approaches his mission and his loyalty to the village could be re - examined through this new lens.
Itachi Uchiha's story is complex. He was a prodigy in the Uchiha clan. He was forced to make a difficult choice to protect the village. He slaughtered his entire clan, sparing only his younger brother Sasuke. He then joined the Akatsuki as a double agent, all while secretly hoping Sasuke would one day be strong enough to kill him and avenge the clan.