What is the definition of a focus city? I guess that it serves O/D traffic from this city to nearby cities. With this said, AUS can be a focus city if it offers nonstop flights to nearby TX cities and a few cities around TX, even though IAH can offer a lot of more connecting flights to these same cities. AUS may become a focus city if UA can make money. With this definition, DCA is just a focus city for AA, as discussed previously.
Just my two cents.