IAD makes a good second or third hub. To really focus on longhaul, you should stick with the big airports like ORD, ATL, EWR, JFK or later opening like DFW or iAH. the bigest issue with ORD and JFK is slots will get really expensive and there's always a blood bath at these airports until things sort themselves out. On the smaller side, you can do well with MSP...but if it's crowded it will be a challenge.