Why wouldn't a Nilfgaardian mage/spy be working alongside Geralt?
It is clearly Emhyr that informs Geralt that Ciri has returned, and he is the one "officially" sending him after her. So I would guess he would send one of his spies along with Geralt, just to be safe. Why would Emhyr trust Geralt anyway?
Sorry for being way off topic.
It's not impossible just unlikely imo. Why should Cynthia wear a mask? Why should he risk to send a spy who is known to nothern sorceresses? I mean wearing a mask in public isn't really a good way to avoid being discovered anyway. If Geralt was in Nilfgaardian territory there is no need to wear a mask and if the was in Redanian/Temerian territory there is also no need to wear a mask. The only similarty between the character and Cynthia is the design of the hood which is probably just copied over from TW2 to TW3. Also the character in the trailer wears bracers which indicates that it is some sort of warrior or at least someone who is ready to fight in close combat. Cynthia is a sorceress and a spy but neither a warrior nor some sort of classic assassin. I don't know the design of the character doesn't fit to Cynthia.