American Express has been building out its own lounge network, and they’re at a much higher standard than US airline lounges.
There are currently lounges in Las Vegas, San Francisco, New York LaGuardia, Dallas, Miami, and Seattle (“Studio”). No additional lounges have been announced.
However I’ve confirmed with airport officials that American Express is in discussions with Denver International Airport to open a new Centurion lounge in terminal B which houses United Airlines.