The Abilene Cowboys.... lol
If the big TV contracts dry up some (advertising revenue is significantly down,) jersey sales tank ($80-$150 for replica jerseys?), corporations quit underwriting luxury boxes, and people realize they REALLY don't have to spend $200 a ticket, $20 for parking, and $8 for a beer, the NFL as a whole could be in serious trouble. Over half of the NBA teams are losing money now. The Buffalo Bills are probably in financial trouble; the Carolina Panthers have attendance problems even when they make the playoffs.
Additionally, I don't think most cities are in a position now to offer a team a billion bucks in incentives to move, so I don't think you'll see any more St. Louis Rams deals anytime soon.