Ciri is an obvious choice IF we want TW4 to share not just the universum but the characters.
Personally I was hoping for the same universum, but new NPCs and completely new story with a character we could create ourselves (a witcher, a sorceress, a human bard...?), but I completely understand the choice of Ciri too.
As someone has already mentioned - Ciri is a guarantee of returning cast of characters. She is the only character that can take over after Geralt without changing many things too much. She knows all the witchers, all the sorceresses, all the kings and otherwise important figures.
In the main story of the books series she is, arguablly, already more of a protagonist than Geralt is... In the way that she has plenty of focus and the whole story revolves around her rather than the witcher, who is 'only' responsible for leading/finding her.
Ciri is a character who still can be shaped as a character. Despite being set as a rightous girl who fights for whatever her heart tells her is worth it, she is still impulsive, too emotional, she still loves mischieves and is no stranger to being simply... naughty... In a sense she is a complete opposition to Geralt, who was supposed to be emotionless - she is a bomb of emotions. Since we are being introdueced to her being a witcher I can imagine she will get to be steered by players between being emotionally a cold witcher and an affectionate human, without being forced to be either way by her teachers.
I believe there were only 3 choices to create a new game: Geralt (who we already knew won;t be the case), Ciri (who was the only option to more or less continue on the path set in trilogy) or a completely new character with card blanche, mostly unknown to all the NPCs we met in trilogy.