I've been rereading the series both because of the HBO release and because I've been playing a lot of BoW. My guess is based off of what I've been reading. Here's a few important points for the reasoning of my own guess:
--Book spoilers follow! --
-FFG's has been following the books in chronological order where it comes to the overall flow of the war even when creating original battles.
-After the Tullys and Clansmen none of the Houses following Stark and Lannister really seem to doing anything major enough on their own to require being added as a neutral faction (during the mid-end of the second book where we are currently). Frey isn't even on Lannister's side and the only major action Bolton has been involved in was either already covered or in the case of the taking of Harrenhal was only a minor skirmish. In other words it's not their time yet.
-In general I'm getting the feeling that the Tullys and Clansmen are fan favorites but any further releases before coming out with one of the other major factions would be considered "stringing players along".
-Renly Baratheon never really takes part in the war and will likely never be covered as a faction for BoW.
-Stannis sits at Storm's End for much of the second book.
-Before Stannis stirs House Greyjoy has already taken the north.
-House Greyjoy fights House Stark. House Stark is most likely the most popular house.
And so my guess is... House Greyjoy. Dun! Dun! Dun! Not the faction I think most would have guessed. But there it is!
Following that release my guess is that it will be House Baratheon under Stannis. Furthermore I guess that each faction will get two releases to bring them up to the variety that Stark and Lannister enjoy. Next year will be dominated by House Greyjoy and Baratheon giving us TWO new major factions and NO new neutral factions.
It is also my guess that these expansions will depart from what we are used to and will mix in this year's expansions miniatures for scenarios as well as the core set. For example it makes sense to have Rodrik Cassel in the north and the Clansmen engage Stannis' forces.
And there you have it. My guess for year two of Battles of Westeros!