I personally found Geralt to be the most complex, interesting, intriguing, believable, and badass character in the books. Some might disagree with me and say Ciri is the best and so on. But I personally wanted to get back to Geralt's story, whenever the books' focus shifted to Ciri or other characters. I wanted to get through the chapters focusing on Ciri as fast as possible to get to Geralt's. I believe Geralt is the character that contributes the most to the greatness of the books.
That being said I would love to play as Ciri if CDPR releases a story DLC that takes place after the events of Witcher 3 or subsequent to the Witcher 3 (experiencing Ciri's perspective, and what we could not see as Geralt), but I can't get myself to play a whole game as Ciri. Same goes for character creation. I think Witcher games are good because of the predefined characters and Geralt.
I just can't see myself reading a Witcher book or playing a Witcher game with no Geralt. And as adamastor said, Geralt being there and not being able to play as him would be like playing a Batman game and running into batman but having to play as Robin or some random character.
These are my opinions, and I think most people here would disagree with them. But if Geralt is not the protagonist of the next Witcher game, Witcher 3 is the last Witcher game for me. I don't see the point of a Witcher game that you play as a random character that you create. Then it's not Witcher for me, it's more of a Dragon Age or Elder Scrolls experience.