I would bet on Kovir and Poviss plus the Hengfors League.
Lore reasons:
Kovir and Poviss was a major player, who could stand up to the Northern Realms. Furthermore, Kovir and Poviss as well as the Hengfors League stayed neutral on paper in the wars against Niflgaard.
In my opinion that justifies them to be an own faction.
Card pool reasons:
Esteradd Thyssen was already a Gwent card once, but isn't anymore. Also, we already have Tridam Infantry and Reavers in the game, which are basically part of KP+HL, but expect the Reaver Scouts, which can be replaced easily by another unit, neither of them has any archetype around them currently, so they could easily moved the KP+HL. The NR at the same time have enough kingdoms directly north of the Yaruga to expend their cardpool with them, without relying on KP+HL.
Also, opposed to Serrikania, KP+HL offers a lot of known names, for leaders as well as for silvers and golds. While it isn't that bad to create a few characters completly new for the game and can be necessary, having nearly a whole of made-up characters wouldn't offer as much remember value as the already existing factions, and is therefore unlikely in my few.
Moreover, with lots of smaller know territories in KP+HL, naming units wouldn't be that difficult.
Archetype reasons:
How many usefull things do we know about KP+HL to make archetypes? Incidentially, quite a lot.
We know that it is really rich, with a lot of ores, even rivalling Mahakam and that Kovir traded with the whole world and might be compared to this worlds venice.
Also, with all their money they rely heavily on Mercenaries to fight their battles and bought mercenarries to fight for the NR against Nilfgaard too.
Kovir and Poviss had a lot of immigarnts from the NR, for various reasons, like escape from prosecution or a life of more possibilities. Even in Witcher 3 the surviving mages from the NR flee there.
I believe this would offer at least 3 possible archetypes.