Are there any American cities that would even want (can afford) an NFL team at this point? Portland is to far left to throw hundreds of millions of dollars at a billionaire. Austin already has a crappy football team so they don't need two (plus the Cowboys would never allow it). San Diego, St Louis, and Oakland already told their existing teams "No Thanks - We're broke". Oklahoma City doesn't have the money either and their focus is on MLS (and the Sooners and Dallas Cowboys would be a big No). Salt Lake City? Maybe. Birmingham? They are smaller market than Jax.
The main markets that always seem to rise to the surface are San Antonio/Austin (one discussion surfaced about a team in between the two cities as they're a little over an hour apart, but I have a hard time seeing that working or many reasons), Montreal (sharing the Alouettes stadium), Portland (though I'm not sure where the stadium money comes from either), St Louis, and San Diego.
You're not exactly correct on St. Louis. St Louis actually was willing to put legit money up for a new stadium to replace the dome. The problem was, Stan Kroenke wasn't really negotiating in good faith. This is why St. Louis is suing the NFL and Kroenke. Multiple legal articles I've read opine that St Louis has a legit argument here, which I bet COULD lead to another team. If St Louis gets anywhere near what they're asking, then I think the numbers will prove Kroenke was better off stay in St Louis, especially combined with the Chargers issues (See below)
San Diego is a complex issue. The Spanos family (owners of the Chargers) could not have botched the PR side of this any more than they did. Further, they were just really stupid with money - between all of the money in studies and the $550M Relocation fee to LA, had they just put that money towards a new stadium it would have been a MUCH more palatable number for the folks in San Diego, plus the league was willing to loan Spanos money to front the costs.
As it stands now, the Chargers can't sell out a 28k seat soccer stadium, and the LA Stadium that the Rams and Chargers are supposed to share is looking like a multi-billion dollar money pit, since the Chargers can't sell tickets to save their lives. Now, there's no public money involved here so the public doesn't really have a complaint in my eyes (Kroenke is writing a check for the thing).
Oakland is also an interesting issue. In Mark Davis defense, he actually DOESN'T have the capital to front the money. Davis is well known to be the least wealthy NFL owner (worth about $500M, but it's all tied to the team). Now, I ALSO think he botched this, as Oakland had a funding plan for a stadium (the public's share was $200M for a $1.3B facility). Bottom line was a reasonable amount of money WAS offered, but I think Mark Davis, frankly, was too stupid to figure out how to make everything else work. I think he'd have to give up some control of the team to another investment group if I remember right.