Warning: There are spoilers hidden in this post.
Grandmikus, I disagree with your assessment of the Brotherhood. They are far more important than the troops of Greywater Watch
Beric and his men not only play an important role in the books two to four (whereas we only know about three people of Greywater Watch), but they fit the Riverlands theme this game has developed so far much better than for example the clansmen-expansion. I'm really looking forward to being able to buy these Merry Men (when they come out in German), and to pit them against the Lannisters - or, for that matter, against Edmure Tully.
But I'm also looking forward to the next expansion featuring one of the great houses. I agree that a "Battle of Blackwater Bay" would be a great introduction for House Baratheon. Maybe they could make it a Baratheon vs. Tyrell expansion instead of Baratheon - Lannister.
Or maybe FFG could combine both approaches: They could give us the Ironmen and in combination with that the troops of Greywater Watch (eg the Crannogmen). That would make for a few interesing scenarios surrounding Moat Cailin, I think.