I know that Alaska Airlines is ordering the 737-900, and I was wondering if Southwest is a potential customer. One would have to assume they were, being that they operate an all 737 fleet. The -900 would open up many new posibilities for them. They could get clear across the country (BDL-LAX, BWI-SFO, MCO-SJC, MHT-LAS) and also would give them trans-continental type capacity without deviating from their concept of al all 737 fleet. In Southwest one-class configuration, they would seat somewhere in the neighborhood of 185 passengers, around the same number as the 757 which is so common on cross-country routes. The -800 may also be a good fit for SWA; it would seat around 165 passengers.
In addition, they could possibly start flights from the US mainland to Hawaii. I think there is stil lots of money to make in that market. Say they bought Aloha Air, which operates 737-200s on interisland flights, and 737-700s on flights from Maui and Honolulu to Oakland. Say WN buys them and transfers their -700 aircraft to the mainland and replaces them with 737-800 or -900s, and expanding mainland-Hawaii flights to SJC, PDX, SAN, and LAS, and PHX, all cities with lots of possible domestic connections on Southwest. With the -800 and -900 deliveries, WN could use some of the smaller capacity 737s (-500s) to expand into Canada (Montreal, Calgary, Winnepeg, Vancouver, Toronto) because with Canadian and Air Canada merging, fares are sure to rise. Of course they also continue to expand their Domestic network with new destinations, more frequencies, and additional routes. Mexico and the Carribean could be explored, but I think Canada is more likely.
With around 310 B737 aircraft and continuous profits, they must have the capital to make a big push into new territory!