Just to engage in some idle speculation: what would be come of AA's hubs under such a combination? My view...

1) Surefire keepers: CLT, MIA, ORD, JFK (I think UA would just absorb all the slots and expand JFK as a bigger spoke and maybe add a few key TATL routes), LAX (UA would surely want to keep any additional LAX gates given current constraints)
2) Unclear: DCA (not sure UA cares to run a huge operation given existing presence at IAD), DFW (unsure of relationship vis-a-vis IAH), LGA (feel like UA may add more frequencies but I cannot see them growing traffic inside perimeter at risk of diluting EWR)
3) On the chopping block: PHL (think would be easiest candidate for dehubbing given proximity between EWR and IAD), PHX (not sure where this really fits in within the broader UA strategy, along with UA having a strong inland hub already at DEN)

Of course the bigger issue is that I do not see how this overcomes the obvious antitrust issues. UA + AA combined would be absolutely massive.