It would be awesome for Ciri to play a big part in a sequel and even be playable, but TBH if she was the MAIN protaganist, I probably wouldn't buy the game.
I would much prefer to be able to create my own character and determine their gender, abilities, etc.
Also I really don't like playing as females. Not that I don't like them or think they're bad. It's an immersion thing. For some reason if I'm not playing as a male I just can't really get immersed into the game. I like to play RPGs to create my own character and make them as close to "Me" as I can, though I usually give them hair, while I have a shaved head in RL haha. So in a way it really is "Me" in that game world, kicking ass, wooing ladies, slaying monsters, and all that other various hero things.
Sure it's probably not right and I'm sure many people will disagree with me, but that's just how I feel.
I think it would be far cooler, if the remaining Witchers and Ciri were trying to get the Witcher schools back up and training more witchers, and altering the witcher mutations so that there's a much less chance of horribly dying or turning insane during the trial of grasses so there's more witchers out killing monsters.
Because, as of W3, Witchers are a dying breed, all the schools are in a state of disrepair, not a single known school is training new witchers, and those that are still alive are either dying on the job or are looking at retirement.
If this keeps up then it seems like there's a good chance of more and more monsters popping and causing a lot of trouble.
The next witcher game can have a cool feature of rebuilding Kaer Morhen and some other schools by looking for people to live nearby. The war has caused a lot of refugees and Velen was kind of a horrible place to live anyways, so it doesn't seem unlikely that Geralt and friends might consider letting some refugees live around Kaer Morhen in exchange for helping to do repairs and work around the school.
It can be like the Assassins Creed games where you have your hide outs and cities, and find people that can make upgrades and improvements.
For example you can find a refugee stone mason, and let him and has family live in the area in exchange for rebuilding KM. Farmers can have farms nearby and give a portion of their crops to the Witchers.
At first a lot of people in the world may not like this, but with few other places to go and monsters popping up in more places, living near a Witcher school of professional monster hunter hunters suddenly doesn't seem like a bad idea.