I'm just looking at all the following Delta hubs online at Wikipedia.
Amsterdam Airport Schiphol
Charles de Gaulle Airport (Paris)
Cincinnati/Northern Kentucky International Airport
Detroit Metropolitan Airport
Hartsfield–Jackson Atlanta International Airport
John F. Kennedy International Airport (New York City)
LaGuardia Airport (New York City)
Logan International Airport (Boston)
Los Angeles International Airport
Minneapolis–Saint Paul International Airport
Narita International Airport (Tokyo)
Salt Lake City International Airport
Seattle–Tacoma International Airport
With Virgin already serving Atlanta, the Delta/Virgin swap on the JFK and the new routes in 2017 to San Francisco and Boston, i suppose the only viable route for Delta to try out, would be Detroit or Los Angeles. I would probably back Detroit.