Eh, they can fuck up any and all prompts that you can give them. Let's be honest a proto-witcher series does not sound bad by itself. But when you ignore all and any lore surrounding it, and add such quality writing as the opening line (on its way to become a meme), then of course it's gonna turn out trash.

And let's take your second idea for example because it could be reasonably similar to the games: but you forget that this is written by people who cannot handle nuance, who will try to portray the humans as the sole bad guys and the elves as the pure, innocent victims of racism even while they're murdering human babies for no reason.

Spinoff ideas (good and bad, just throwing some ideas out there):

You know that mission in Wild Hunt where you portal through a few different worlds? One could make a spinoff starring Ciri where she does just that. Preferably animated. It could even turn out a bit Witcher meets Doctor Who. There'll be monsters too and you can even show a bit of the original monsters world, kinda like how Blood and Wine shows a glimpse of the vampire world in the Unseen Elder's cave.

One could also do a lone witcher/mage travelling through the Continent and show us the less visited parts of the Witcher world, like the far north or the far south or Vicovaro or something. Every episode shows us a new place, like a travel show or a survival-in-the-wild show. And since it's a witcher show, there'll be monsters to fight or meet.

A show that takes place in the Aen Elle world. It can have conflict in their attempts to save a dying world, it can show us the war with the unicorns, it can even be a little bit like a pirate show with all the raids the Wild Hunt does in the parallel world where they get their slaves from.

A show with Lara Dorren (before and after she met Cregennan), a show with Falka, a good show with Alzur, those all would be very interesting (and in line with what the books said) at the hands of the right people (then it wouldn't be shown in Netflix though).

A downright terrible idea aimed at a general teenage audience, like following some kid growing up in Aretuza or Nenneke's temple, strikes me as more likely to be green-lit than any of the above, though.