WN also flies into different airports at some major hubs. It uses HOU not IAH and MDW not ORD. Where UA is domestically weak - a hub like IAD, WN offers no value.

For California, both use LAX and SFO, but they compete directly against each other. I can't see WN interested at all.

A better partnership might be with JetBlue. Would it work though? I could see many some synergies in BOS with JetBlue having domestic feed maybe for some United international flights if United offered them and Star Alliance partner flights. Would there be benefit if JetBlue joined Star Alliance and benefit for both United and JetBlue?