Mmm...Wild speculation. That's what FT is good for
If we make the not so wild assumption that the DL/NW merger happens, it is clear that they will remain in ST. When the discussions were about DL/UA there was a comment made that UA would consider leaving *A, but I do not think that would happen in a CO/UA merger (which is not a foregone conclusion, but reasonably probable) following NW/DL. Having 4 USA carriers in ST is overkill and wouldn't be productive for the alliance.
That being said, I would not be surprised if the DL/NW combined entity decides to expand dramatically into South and Central America from their ATL hub. I also wouldn't be surprised to see them work on expanding LAX more and even possibly scaling back SLC, and that LAX growth could be south-bound.
CO doesn't want to merge unless their executives can remain in charge and unless they can be reasonably assured that the merger will actually improve their competitive stance in the marketplace, which is probably a 50/50 bet these days.
S.