The favourite contenders would be Cyndane/Moghedien whom Graendal implied were sometimes in the south trying to carry out Moridin's orders concerning Perrin/Mat. We've seen some of that with Mat in TGS, but if that's not the unseen threat then the synopsis makes no mention of this threat...

Since Perrin's story line will deal with TAR, it could really be either. It will be Moghedien if Jordan wanted the "unseen threat" to be something Perrin has never faced before, or if it's the opposite (and it sounds like everything in Perrin's story line is centered on old stuff returning) it will be Cyndane. Cyndane sounds more likely, I think, because even all the way back to TDR, she was involved with Perrin and tempting him to seek leadership and glory while Ishamael was trying to convince him to give up and go back home to be a blacksmith again. His struggle with leadership is mentionned as a theme for ToM. Furthermore, the other ta'veren Moridin wants dead is in Caemlyn, where Moghedien was active for Moridin before, and her backstory involved Elayne and Birgitte (and Mat, very indirectly - he met her agents in Ebou Dar). So it sounds very logical that Moghedien's part is with Mat, and Cyndane's part is with Perrin. Her new body might not fool the wolves who might still call her "Moonhunter", by the way. An involvement of Slayer and Lanfear is a natural fit too. It's highly suspected she's been the one who used his services a few times (to get rid of Ishamael's greymen sent to kill the girls she wanted to use, to kill the BA in Tear if Elayne's theory about them is correct: she thought the Shadow had them killed to convince them they had revealed something important and to leave Tear after that information. The one with a motive to make the girls leave Tear and Rand's side would be Lanfear, who was about to launch her plans with Asmodean and to seduce Rand to her side).

Fain is another option for the "unseen threat", but usually Fain doesn't remain "unseen" for very long. I don't really see what he'd be looking for in the middle of nowhere like that. He's far more likely gathering followers again, probably near a city (for myself I think there's a reasonable chance from Far Madding he fled home to Lugard, where he found that Roedran was becoming interesting as a pawn now that he had an army).