With the Witcher universe not being incredibly massive to support an infinite card pool I think about what hasn't been added to the game that could be. They aren't afraid to make cards, or even legendaries, out of small characters from Witcher 3. Looking at you Tinboy!
Help me brainstorm, here's some that I can come up with. Mine are mostly from the games because I haven't read past Last Wish *yet*
Sigi Reuven's Gang – His stylish henchmen could get a few cards
Happen – Bathhouse Eunuch
Bart – The best Troll
Renfri – Because that custom card guy would be happy
Var Attre Twins – My Geralt was bummed he couldn't plough Rosa
Major Domo – BB!
Folan and Vigi the Loon – They maybe fought at Kaer Morhen! They should be in already.
Hjort – More druid support
The La Valettes
Bernard Loredo – Chump from Witcher 2 I barely remember but he was important in his chapter
Bankers – Vivaldi and Cianfinelli
Another Siegfried card cause he's my bro. Giving him a Firesworn card doesn't make him bad.
Missing Leaders – Henselt, Demevend, NR Adda, Hemmelfart, Bran, Calanthe, Svalblod, Gernichora, Ardal, Gundrun, Syndicate Francis Bedlam, Filvandrel.
Witcher 1 and 2 are pretty well covered. I don't think any of these are deep cuts. They were more memorable than a lot of existing cards… The bear from the whoreson arena Boris has his own named card. If they start adding in fist fighting opponents I'll be worried.