As crazy as it sounds, there isn't enough demand for another football league.
The main problem - when would they play their games? No new league is going to have enough demand to draw viewers away from the NFL or NCAA. This would force all the games to play either during the offseason (as a few of the startup leagues have done) or in the middle of the week. Add that to the inherent advantage the North American teams would have over their world counterparts; virtually all the stars are from the US, all the coaches, players who get dumped from NFL teams would wind up playing for the US WFL teams, etc.
Schedule making is complicated business, but they can in fact make a schedule work with a strange number of teams. There haven't always been 32 teams in the NFL!
In the 70's, they had 4 divisions of 4 and 2 divisions of 5. Starting in '76 they switched to 2 divisions of 4 and 4 divisions of 5. Before the Texans came into the league, they had 5 divisions of 5 and 1 division of 6.
If they choose to expand - I doubt they will soon, as a team should be more likely to move than to add more teams - they can make it work with a weird number of teams. 36 might be an ideal number, as each conference would have 18 and there would be four 5 team divisions and four 4 team divisions. Scheduling would be a nightmare on the surface, but all they really need is one consistent method to make the schedules. They have plenty of people from the league offices that are probably working on it before they even announce the possibility of expanding.
I want to take a minute and plug in
this website. It contains maps of the US showing where all the games will be broadcast, for those of us without a Sunday package. It definitely comes in handy to see what out of market games you get for the week!