I'd like a little more Witchering and a little less end of the world situation if they start Ciri's solo saga.
Ciri would make a terrible protagonist in a Witcher game and any RPG game in general, and it would be a miss step for the Witcher franchise to have her replace Geralt..
Ciri is not mutated, she can't drink Witcher potions, she doesn't use Witcher signs, and she is not an isolated freak by any means like the rest of Witchers. She is a beautiful elder blood superweapon that travels through space and time, and it makes her a terrible candidate for being an RPG or a Witcher protagonist. She is not a Witcher just because she was trained to fight like Witchers with a sword, because a lot of the Witcher 's essence comes from Witcher being mutated freaks who look different and are disenfranchised and even despised. She also lacks in every aspect compared to Geralt including the personality and witty and dark humor. Plus two of the three endings eliminate the possibility of a continuation of the franchise with her, and there really is no reason to continue past the Witcher 3 for either Geralt or Ciri.
Instead I want an action adventure spin off with Ciri as the protagonist, which would fit her abilities and character perfectly. As far as Witcher as an RPG goes, I sincerely think CDPR should keep Geralt the protagonist. Geralt has become well liked by the fans and new comers alike, and he is now practically the face of the Witcher series.
There is so much more to explore with Geralt as far as prequels go. CDPR can start from the beginning, and show us the trial of the grasses and the first time he leaves Kaer Morhen and goes on the path. They can also give us a prequel set during the time Geralt was chasing the Wild Hunt the first time around in order to free Yennefer, and the game should cover his journey, his time riding with the Wild Hunt, his escape and memory getting wiped,. There is still so much to explain as far as the Wild Hunt goes, and this would be the perfect opportunity to learn more about them through Geralt's eyes as he pursues them. The lack of development for the Wild hunt was basically the major flaw of the Witcher trilogy, and another game can fix this and clarify many main elements of the story including Geralt's time riding with the Hunt.