The idea of hooking up with US would seem to have much more potential by having much less overlap.
I believe AA/OW is F9's best bet as far as a major alliance is concerned.
Given UA's existing presence in DEN, F9 offers very little out west for UA/CO. For that reason, a US partnership would be out of the question. US
would not partner with F9 and bypass its far stronger alliance partner.
AA could probably better rationalize a partnership with F9. At least they would gain a regional network they currently dont have.
MKE is too close to ORD to bring any real value to either AA or UA.