I'm still not convinced Ciri cannot be a witcher. Yes, there is 100% mortality rate on girls who are undergoing Trial of Grasses, but even if this may sound as a little "lazy" explanation (and I admit, it kind of is, unless it will be expanded in a way that makes it more interesting), I'm still not buying Ciri's blood cannot make her resilient enough to survive it. I also can't see why the idea there was an ancient school of witchers who perfected the formula, to work on Ciri (not necessarily on all girls). Maybe all they lacked was a testing subject (one is not seeing a woman with an elder blood everyday).
I agree with @Bruce_O there are traps here, waiting for developers, of reaching for oversimplified explanations, that would make the whole thing unbelievable, but CDPR writers has proven to us many times already, they are masters of their craft. Let them cook.
As for Ciri being a protagonist, I admit I wasn't happy at first, but not because I have anything against her, or more broadly, anything against playing a female character. It's just seeing how well CDPR performed with CP2077 protagonist, I cannot see why this could also not work in TW4. I simply hoped to play a custom witcher, with a fixed background (like V or Shepard from Mass Effect). But now I'm soaking the idea of playing as witcher Ciri and I see now, it was always meant to be Ciri. I just hope she will much more customizable than Geralt, also visually.