Looks like Boeing is shopping a new version of the 737-900 with two additional exits and extra fuel tanks so that it can actually carry more people then the current model the same distance as a 737-800. Article here -
http://seattlepi.nwsource.com/busine...eingjal07.html.
We know the current 737-900 has not been a good fit for AS, which is why they are swapping orders for the 737-800. AS could put around 204 seats (say 24F / 180 Y) in a 739X in a two-class config vs. the current 172. And it would have the range to do the SEA and LAX transcons.
Frankly, this is what Boeing should have done in the first place with the 739, but that's water under the bridge.