LAX-MEL is already served nonstop by partner airline Virgin Australia with a 777 every other day. That route also has a daily nonstop Qantas A380 and a daily direct (stops in SYD) UA 747. I don't think DL is going to throw their hat in there.

JFK-EZE could be interesting now that AR is in SkyTeam. AR only goes as far as MIA. AA and LA already operate this route though, and UA operates JFK-EWR.

SEA-LHR would be weird. Presumably like most of the DL international flights from SEA it would rely on a fair amount AS feeder traffic. AS already feeds the BA flight to LHR through their partnership with AA. Feeding a competing DL or VS flight could be challenging, not to mention awkward.