The Witcher 4 devs explain why Ciri had to be the RPG series' next protagonist: "from the very beginning we knew it had to be Ciri"
Ciri is described as "Geralt's opposite," due to how sociable and outgoing she is, but also due to her naivety compared to Geralt's seasoned skills. Of course, she can also wield magic - something explored in both The Witcher 3 and Andrezj Sapkowski's novels.