This topic came up in my Finance of Sport class this past fall. Iowans would fill the stands without too much of a doubt. Most professional stadiums only hold 65-70K anyway.
Obviously the two main hurdles would be television and corporate dollars. Not only advertising/naming for a stadium, but a HUGE portion of season ticket holders at the professional level are corporate tickets.(especially in the premium seating sections) So while fan support would likely be there, the finances and television incentives simply wouldn't. And I'm not sure if Iowans could fork out enough money to compensate for the lack of corporate season tickets.
The best chance the state of Iowa would have of hosting professional football would be a minor league franchise like the D-league. Two NFL franchises sending down players to one minor league affiliate. Smaller stadiums would be needed as well. Because it is football and because the number of games would be 6-8 home dates, the fan support would be better than the D-league or AAA Iowa Cubs.
I don't see that sort of opportunity happening for many years, if ever. The NFL would never happen. However, if Iowa had an NFL franchise from the beginning like they did in Green Bay, I DO think Iowa would have been able to support the franchise.