TBH, I think saying that Geralt returning from his usual "contract hunting" and of course the situation he is left in at the ending is unrealistic is in and of itself unrealistic.
This is a very grim world, Geralt has made a huge amount of enemies, and possibly in TW3 very powerful ones too, depending on your choices... It would be child's play to create another game with Geralt in it at the very least, though making him a protagonist would take considerable skill and talent as a writer without making it seem as a cash grab
But look... sometimes, a silly reason / excuse to use a character again is well worth it
But no Ciri would not work as a protagonist at all, for so many reasons, namely her immense power being put against how a game needs to be challenging to this fan base especially, in order to be fun
All in all, if they do make another Witcher game, it NEEDS Geralt