WARNING: SPOILERS for the whole series
Rumors on the street are that Matrim Cauthon is going to be involved in the Tanchico storyline in Season 3. Some people have suggested that they are going to combine the Tanchico storyline with the Ebou Dar storyline from the books. This makes little sense to me unless they truncate the plotlines for all of the other major characters as well. This is driven (IMO) by the possibility that the show will not be renewed for a full 8 seasons, which has not yet been greenlit. If Rafe is told to wrap it up by season 4 or 5, then they will need to move quickly, and combining Ebou Dar with Tanchico will make it easier to do so. I really don't think we are going to get Mat in the land of the Finns.
What will happen in Season 3? I think that these things are reasonable to assume:
1. Nynaeve, Elayne, and Mat in Tanchico. Find ter'angreal stash with the bowl of the winds (domination band maybe?). Nynaeve overcomes her block, gains a nemesis in Moghedien
2. Rand is in the waste. He does the Rhuidean thing, gets the majority of the Aiel on his side, defeats one of the male forsaken.
3. Siuan is deposed.
4. Perrin returns to the Two Rivers and defends it from Shadowspawn and Whitecloaks. He meets Faile, who intended on hunting for the horn in Manetheren.
What happens next in a potential Season 4? Cairhien is not at civil war in the show and has a stable government (as of the end of S2). Caemlyn has not yet made an appearance, but apparently Morgase has been cast for S3 so we should see it at some point. Sammael has apparently been cast. Is he Lord Brend in this universe? Or does he take the role of Rahvin? I think that Rand will take the Aiel over the Dragonwall early in season 4 and either take the Stone of Tear or conquer Cairhien.
Will Egwene be in the waste? She really doesn't "do" a lot in the Shadow Rising besides tag along. We know that she is Rafe's favorite character, and her storyline is often prioritized by the writers. Could it be possible that she will be involved in the tower storyline in Season 3?
Unknowns:
What is going on with the Dark One? Is he bound by seals as in the books? The ones keeping the Forsaken imprisoned were all destroyed by Ishamael at the end of season 2.