Chicago would be interesting. There is already shuttle-like service from UA and AA and actual shuttle service from DL. Similar to DCA I'm not sure it makes sense to enter the rather saturated market, even with the high number of passengers. Oh, and I'm not so sure about the B6 gate situation at ORD.

The slot swap between US and DL will also shake things up. But I wouldn't necessarily only expect to see them go after top 10 markets.