After hearing about the merger announcement I initially thought why would Doug Parker not seek Northwest as a partner instead of Delta if for nothing else less route overlap and a more similar fleet. After at looking at Delta Closer, the fleet's are more similar than I originally thought.
US Airways and Delta both operate B767s, B757s, and B737s. The two main differences are the MD-80 and B-777s and the A320's and A-330s. Delta operates 8 B-777s and US Airways 9 A-330s, thus, according to Parker, one widebody fleet type would be replaced.
In my opinion, with Delta's recent B-777LR order, with deliveries scheduled in 2008 and Airbus' continued widebody problems and US Airways' A-350s not scheduled to arrive until 2014, the A-330s could fairly easily be replaced by B-777s, which would eliminate one fleet type.
In addition, due to Delta being in bankruptcy, all of the MD-80s could have thier leases rejected on an incremental basis and returned to the lessor during or after bankruptcy, similar in scope to what US Airways did with its B737s.
These aircraft could then be replaced by either A-320 family aircraft and/or B737's, with US Airways close to announcing a narrowbody deal with either or both Airbus and Boeing in the not-so-distant future.
This could leave the combined carrier a fleet of:
B-777s
B-767s
B-757s
B-737s
A-320s (family)
EMB-190s
A problem with the 757's is that DL's are P&W powered and US/HP are RR -- DL has 100+ of them.