DL-AA wouldn't ever happen - there is way too much rivalry there. The only reason that UAL-US is happening is that they had a deal before and United is in a spendhappy mood these days.
It's true that DL-AA would be easy from a fleet perspective, but look at what would result:
Hubs: CVG, ATL, SLC, LAX (sorta), ORD, MIA, JFK, but the real problem is DFW: talk about a fortress hub now... UAL-US aren't doing this (having a merger with a common hub). Can you say antitrust? AA is already in it deep with Vanguard at DFW. The only way this could happen is with major divestiture at DFW, which could kill the deal in and of itself. Plus, there's still little penetration (except the shuttle) in the Northeast.
Int'l: AA strong to Europe/Latin America, DL strong to Europe/Latin America. Still no vision for Asia, where the growth will be.
That combo, while formidable domestically, wouldn't really do anything else. US-UAL (which is a strong airline taking over a relatively weak one) accomplishes more.