San Diego is a miserable sports town. Great place to visit. I would never put a soccer team there. Only saving grace is that soccer is a millennial sports and they have a lot of them.
St. Louis is a great soccer town. I would like to see an MLS franchise there.
Las Vegas is a hot place for poor retirees. Awful city for the MLS demo.
When looking at the expansion of MLS, you need to consider what MLS 2.0 is all about - revenue. The new sources of revenue will not be from the gate receipts at a sold out 18,000 Indy Eleven stadium. The new revenue source will be from having new television markets opened up.
That being said, Indy and Sacramento should get teams.