First, we know that Taim is now on the dark one's side. Personally, I think that Taim is Moridin is Ishmeal, but that is another argument. The AM have already attacked rand, and we know that there is a faction loyal to taim that are making trouble for Rand. They are already fairly close to Caemlyn (not that it matters with them) and they know some of the connection between Rand and Egwene. They have motive, opportunity, and capability to attack Caemlyn. I would also like to point out that the other two options (Seanchan, Shaido) are not really reasonable. The Seanchan have been taking the land bit by bit, and they are still a fair distance to caemlyn. The shaido are still to split up to really mount an attack on an army.