Initially, I thought that the new Frontier would complement the DL route system well. DEN and MKE were points without major DL presence (I know NW has been on and off over the years with MKE, but in recent years it's been off) and I thought SkyTeam would want to leverage that. However, as we saw, DL must have been nervous about the proximity to SLC and MSP.

Now, with all the rumors about OneWorld, I would say that American would look at DEN and MKE and much like DL's analysis conclude that there isn't any difference between new Frontier and the ultimately redundant STL hub. I think AA would look at DEN and MKE as threats to their hubs at DFW and ORD.
As an AA flyer I'd like to see it, but from a practical standpoint I don't see it.

The idea of hooking up with US would seem to have much more potential by having much less overlap. But getting *A to agree to anything right now might be difficult with the UA/CO merger. B6 also offers an interesting hookup, with compatible fleets and non-duplicating routes. But they don't have the extensive international network that BB seems to want to hook up with.