I would try to think of airports or city-pairs where DL faces aggressive competition, entry of a LCC, sales by another airline, etc. For example, MKE or the four current "magic" cities of RDU, PIT, BNA, and STL would be a good place to start, perhaps also IND or SEA. Also try cheaper airports such as MDW rather than ORD, alternatives to Boston (PVD, BDL, New Hampshire).