CSAs or probably MSAs are a better measure unless you think only people within the city limits would go to games. San Antonio is the 25th CSA behind Portland, Charlotte, Orlando (24th MSA). Austin isn't in a CSA but its MSA is 28th. So none are slam dunks but they probably could all support something if the conditions were right (Portland and Charlotte seem like the best to me given their wealth and the distance they'd cover -- Austin would be pretty close to Arlington which is likely why they aren't mentioned much).
https://en.wikipedia.org/wiki/Combined_statistical_area
As I noted the LA and NYC areas could likely support an additional team and there should be 2 in the greater Bay Area, but the second should probably be in San Jose. Chicago could maybe have a third but Milwaukee is close.
You don't need to do this all at once but it would be nice to see some action in the direction of expansion. If the thinking is that baseball is more a regional niche game that cultivates loyal followers who drive ticket revenue instead of the National Pastime it's a little weird that MLB is shutting down minor league teams and not considering expanding its reach.