There certainly are fifty new destinations within reach of a 737 (and easier, a MAX 8) from WN strongholds in LAX/PHX/HOU/FLL (and more). Just look at a JetBlue route map of the Caribbean - most anywhere B6 can afford to use an A320 or E90, WN can take a 737.
http://www.jetblue.com/WhereWeJet/
Look at every airport in Mexico that gets mainline AA/US/UA.
Look at every airport in Canada that gets year-round service on WestJet to the Southwest or Florida.
HNL/OGG/LIH/KOA.
The problem isn't lack of opportunities - the issues are boldness to try and the ability to carry it out it foreign markets. As my examples show, they've been beaten to market time and time again.