There's a fair few things we know by now about the upcoming The Witcher Netflix series. We know, for example, who's been cast as the production's version of Geralt of Rivia, which is kind of a major thing to announce.
We can also surmise at this point that the show will likely stray further away from the source material than fans are probably hoping it will. There are a few reasons to believe this, most of which can be sourced back to the casting choices for Ciri.
You're probably aware of this by now, but producers are looking for "a 16 or 15 year old BAME (black, Asian, minority ethnic) girl who can play down to 13/14," which has major implications for the plot and setting of the story that's being developed here.
Namely, there are a few reasons for Ciri being white in the source material: firstly, her race dictates the race of her genetic lineage (ergo, the racial makeup of her kingdom), and her being white makes the plan of hiding her away as Geralt and Yennefer's daughter actually viable.
Now, since a fair portion of the story's plot depends on these things, and Ciri will likely not be played by a Caucasian at this point, we ought to expect some major alterations to the show's story and plot by Netflix. Can it work? Of course, but it's probably going to be different than what most of us are hoping for.
Additionally, we also know that the show will be shot starting 18th of October, with the finalisation planned for the end of May 2019. This suggests that, should The Witcher come out in 2019, it'll definitely be in the latter half of the year.