There are alot of cities that can come up with a good arguement to have an NFL team. The question now is, besides LA, who else gets the nod.
Some potential candidates would be:
San Antonio - was a possible relocation site for the Saints. Nearby Austin is another large, unsaturated pro-sports market.
Portland - A group led by auto giant Scott Thomasen has quietly coveted the NFL for awhile. Portland is another un-saturated pro-sports market.
Salt Lake City - rapidly growing with a diverse economy, Salt Lake City will land another major sports franchise in the future. The potential con with NFL is devout mormons will be unable to attend Sunday games.
Las Vegas - Large enough, but most of its population has relocated there from elsewhere, so there isn't as much local loyalty there. Also mediocre corporate base dominated by the gaming industry.
Louisville/Memphis/Birmingham/Oklahoma City - Any of these smaller cities could support NFL if they can build a stadium and get local corporate support behind the sucker. NFL will kill NBA in Memphis and OKC.
Columbus - Large enough, good corporate base, unsaturated, but loyalty to other teams may hurt a prospective NFL team there.
Sacramento - Large enough, but they are about to lose the Kings. Would take a totally private effort. Would also be helped if the Raiders moved back to LA leaving the huge NorCal market with just one NFL team.
Orlando - Large enough, but too close to Tampa Bay. Would also poach away potential Jaguars fans.
Hartford - Almost landed the Patriots. Large enough, by loyalty to the Patriots and NY area teams would hurt the prospective team. Would instantly become ESPN darling, a mega con against Hartford in my book.
NFL is by far the easiest of the Big 4 to sustain. Because of the small amount of games, primarily on weekends, a regional concept could work (sort of like New England, but on a smaller scale). For example placing a team in Williamsburg, VA which is in between Richmond and Norfolk/Hampton Roads.
Other than the Oakland Raiders, there are no real failing NFL markets. The NFL could domestically expand to 40 and be viable.