Lex is correct - DL + CO nearly happened. CO linked up with NWA after that deal fell through - a move that CO has regretted ever since.

DL + AA is the one that was intended to scare the bejeezus out of the DOJ. DL and AA met together to discuss what was happening (with UA and US having just announced their plans) and leaked the possibility of a AA-DL merger just to give the DOJ a nice 'what if' scenario to chew on.