Before Nightmare of the Wolf, any fans of The Witcher series who hadn’t read Andrzej Sapkowski’s source material had every reason to think that maybe Season 2 would be the beginning of Ciri’s journey to become a witcher.
No more witchers with their particular set of powers can be made.
Coming into Geralt’s custody, traveling to Kaer Morhen, and training under the surviving witchers seemed like the right recipe for beginning the transformation… except that Nightmare of the Wolf pretty definitively shut down that possibility by killing off the mages who made the Trial of the Grasses possible without leaving records of their craft behind.