This is an interesting guessing game.

If it’s a non-stop to East Asia from the US Mainland, BKK makes the most sense. It has a good mix of business traffic, cargo, and year-round tourism. It’s not served non-stop by anyone. Plus, UA’s current routing through NRT (NH connection) doesn’t allow same-day connections within Thailand in either direction. This puts them at a disadvantage to EK, TK, QR, etc.

SGN is the only other market I can imagine. The passenger makeup to SGN is quite different than BKK, however. SGN is more about visits to friends and relatives and less about US-originating tourism. It’s not as premium either. SGN looks more like MNL, which UA has been successful at.

SGN and BKK were both previously served by UA, too.