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.
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.
Naruto would adapt to Itachi's life in several ways. Socially, he'd have to get used to Itachi's connections and rivalries. For instance, his relationship with Kisame would be different as Naruto. He might try to influence Kisame in a more positive way. Emotionally, he'd have to deal with Itachi's internal struggles. Itachi had a lot of guilt and secrets, and Naruto would have to either come to terms with them or find a way to resolve them. And in terms of combat, he'd practice Itachi's signature moves over and over to make sure he could use them effectively in battle.
Itachi's personality could become more outwardly friendly. In the ninja world, he had to keep many secrets and be aloof. But in the wizarding world as Harry Potter, he might find it easier to let people in. He may also be more assertive in a different way. Instead of silently following orders like in his past life, he could take the lead in situations, using his own judgment based on his past experiences and newfound magical knowledge.
In this kind of fanfiction, Itachi might use his powerful Sharingan abilities. He could perhaps create illusions to mislead Orochimaru and his minions, then quickly snatch Sasuke away. For example, he could make Orochimaru think that Sasuke is in a completely different location while he secretly approaches and rescues him.
At first, it's likely to be very awkward. Naruto is a very open and energetic person, while Itachi is more reserved. But as they are forced to spend time together because of the arranged marriage, they start to find common ground. For instance, they both care deeply about protecting the village in their own ways.
Well, Geralt might die as a result of a powerful curse or magic. Maybe he angered a powerful sorcerer or witch who cast a spell on him that he couldn't resist. This could be a slow and agonizing death as the magic takes its toll on his body and mind. Another possibility is that he sacrifices himself for someone he loves or to save the world from a great evil. He might put himself in harm's way knowingly, and in the process, lose his life.
Superman's death in comics is often a result of epic battles or selfless acts. For instance, he might face an opponent whose power is just too overwhelming, or he might give his life to prevent a major catastrophe.