Perhaps less likely than CVG because I'm not sure a 757 has the range year-round but there have been many rumblings in IND of a direct IND-LHR flight.
Incentives have been bumped up big time and I could see DL trying to make an IND-LHR run work 5x a week. That said, I'm sure there are other markets IND is considering, but many people I've spoken with at the airport and at the Chamber of Commerce seem to think an announcement could come by year's end.
My thinking was AA would be the one, but I'm not sure they have open slots, and without a lounge in IND, would be a trickier proposition. Who knows--but just adding my two cents of wishful thinking
