Some thoughts from an FO,
In Oct '04 the company put out a new flight schedule that improved efficiency of aircraft & pretty much took away the need for a new domicile in '05. It was said at the time that prior to that point the 3 cities based upon the study at the time that were the most attractive were LAS, BNA & TPA (not necessarily in that order). It was also said at the time this efficiency move would allow the company to delay until "at least" '06 the need for another new domicile.
SWA generally uses the following criteria for deciding when a new domiciile is required:
They look to have on average 18% of all aircraft terminating in domiciles. This number allows the percentage of 4 day trips to be at the 20% level or thereabouts. The union has a slightly different view of that formula but according to management this is the criteria they use. If that number starts to dwindle below that "magic number", then the 4 day trips tend to become too plentiful & the inefficiency & "pain level" increases.
It costs $1m to open up a domicile just for chief pilots, add nearly another $1m for infrastructure costs, add annual expenditures & recurring costs of salary & staff contributes nearly another $1m....add in moving costs for folks to a new base & the costs really become much higher. The company will determine from a dollar amount when these costs are justifiable....I'm told right now that is not on the horizon anytime soon...things can always change.
As for mini-domiciles....they have been proposed, there are some aspects that appear interesting....again dollars & efficiency will be the determining factor on whether it occurs or not...never say never....hope that helps.