I've been doing some research about the lack of IAD domestic service, similar to MXP in Europe until U2 expanded there, and I think that if F9 has some patience with making routes from there work, IAD could feasibly become a larger station for F9, about the size of CVG, LAS, or MCO. There's plenty of gate space at IAD during non-peak hours, so space wouldn't be an issue, the only problem I can think of is if there's enough room in the market for them, with the 600-pound gorilla at BWI known as WN and NK having a not-insignificant operation there either, and AA in DCA, and obviously UA in IAD. Of course, if UA pulls the same stunt they did to kick out Independence Air, the idea is toast, but I still think that F9 expanding is an interesting idea and something to consider.
Here's some of the markets that F9 could add. I chose a mixture of high-demand routes from IAD and trendy F9 markets that they don't already serve from IAD. For a frame of reference, they currently do AUS, LAS (though it's ending in August), MCO, SAT, and TUL year-round and COS and DEN seasonally. Also note that some markets are meant to be seasonal, namely SAV, CHS, MYR, and PWM in the summer and PBI and RSW in the winter.