Last I checked, Delta did not fly to Australia. I suspect that our best chance for service down under is via HNL, IAH or LAX.
I have this sneaking suspicion that a merged US/DL might not be allowed to have a bilateral relationship with any other US airline, precluding it from a global alliance. It will be tough enough for either combination to pass the anti-trust folks.

SkyTeam
DL/US/NW/CO/AS

Star
UA/DL/US

OneWorld (highly unlikely regardless)
AA/DL/US

Now...if CO or NW were to switch sides, all bets are off. In my humble opinion for us CO flyers the best case scenario would be for CO to join Star without merging with UA. Would solve many strategic problems for CO including LHR, Asia, a better west coast feed to CO Mike, etc. without the problem of combining two different cultures.