seekerpat: In TW2 Geralt saved The Northern Kingdoms (more or less-exposed the Lodge's conspiracy, maybe staved off a Nilfgaard invasion, saved the Dragon)
I left Kaedwen without a king, saved a friend rather than saving Foltest's daughter (And thus stabilizing Temeria), and killed the dragon. So I would very much avoid claiming that I 'saved' Temeria.
But yeah, I agree with you that the third game will almost certainly feature the Wild Hunt/Emhyr of Nilfgaard as the primary antagonists. But I disagree with your speculation as to their precise role.
SPOILERS FROM THE BOOK - SPOILERS FROM THE BOOK According to the Ithiline Prophecy, the child of Ciri (Daughter of the Nilfgaardian Emperor) will rule half the world, and this child's child will rule the entire world. Therefor, the Emperor has been trying to find and impregnate Ciri - his own daughter.
However, the leader of the Wild Hunt has also set his eyes on the same prophecy, and is also chasing after Ciri, with the intent to have a child with her.
Due to the Wild Hunt's involvement in Geralt and Yennefer's (Both of whom were like parents to Ciri) memory-loss and abduction, I've come to think that perhaps the Wild Hunt took them in order to lure Ciri to them.
On top of that, the Emperor might have a similar plan, as Yennefer (And soon, perhaps, Geralt as well) is already a 'host' at his palace. There's too little information about the next game to predict any further than that.
I invite any Polish readers of the books to correct any lore-wise mistakes I may have made.