All AA hubs have service to LHR, plus RDU, MSY, AUS, SJC, SFO, ATL, SAN, IAD, BWI, BOS, DEN, EWR, LAS, SEA, PDX, SLC, DTW, and IAH.

That leaves CVG, CLE, IND, STL, CMH, and PIT without service to LHR. (MCO/TPA/FLL/OAK have LGW)

Although I am biased, I personally see IND as being the one because AA wouldn't have to worry about WW/FI/e.t.c eating into yields(unlike STL/CLE/CVG/PIT). And, as I mentioned earlier, IND is the largest unserved US destination from Europe and has high average fares as well. Subsidies are ready to go also. With CVG DL already has corporate contracts with the major employers, and with PIT I'm hearing Aer Lingus is planning to announce flights soon so I doubt there is space for EI/DL/WW/DE/and AA to all go TATL from PIT.

Presumably, AA could work out contracts with Rolls-Royce/Cummins/Eli Lilly/and or Roche for cargo and or pax...