The Rats could be interesting as cards! Something revolving around their thievery could be fun to play with in a deck. One of the things that I've liked as a tie more between the books and the Witcher 3 game and Gwent is the way the faction special abilities seem to be in keeping with the groups in the books:
Skellige: Battle-ready and resilient people, who tend to somehow gain strength as fights go on == faction ability of gaining base strength as rounds go on.
Monsters: Where there's one, there are likely more, and they like to linger despite adversity in an area == keeping a card at the end of the round.
Scoia'tael: Obviously, the card feature of ambushes, but in the books and the games, they were known for turning the tide with offensive positions and feigning defensive positions when they were really on the offensive == choosing who gets to go first.
Northern Realms: Value placed on monetary things like trade and commerce == point buff for gold cards.
Nilfgaard: Emphasis on the spying, of course, but the changing of players in any given situation and not knowing exactly what's going on == ability to change cards each round if desired.
I would love to see Villentretenmerth reworked to be closer to the overall character in the book. That was probably my favourite 'side-story' in the books. Not quite sure how they'd work it outside of a transformation of sorts, or even add a card that is more in keeping with the 'can assume any shape' aspect of the character rather than his power in the books as a very strong fighter. The current version makes sense in terms of Borch, Three Jackdaws revealing himself to be a Golden Dragon and laying waste in his story, but it would be neat to see another, probably gold card, that could assume the points and properties of any other card on the board. There are ways to counteract that with the same cards you'd use to counteract the current incarnation. That's a thought to bridging the character and the game though. A more powerful version of Dudu, which is fitting, since Golden Dragon.