It's really all conjecture because you can't just add airline A's numbers to airline B's numbers and have a result. Places where significant overlap occurs would require significant divestitures by the government to gain approval.
That's one of the big reasons DAL & NWA like each other, because they believe that they will be forced to divest virtually nothing because of very little overlap.
In contrast, as an example, if USAIR had been successful in it's hostile takeover of DAL, one of the New York shuttle operations (probably DAL's) would have went bye-bye. So you'd be merging just to turn around and sell off a highly profitable section of your business.
-If UAL and NWA were to get together, big problems with the Asia overlap
-If AA and CAL teamed up, big problems with south of the border overlap
-AA and UAL, Chicago overlap
-DAL and CAL, New York overlap
Things like that