I tend to agree with Mark on this. Adding teams in England just won't happen. Not that it's impossible, just that it's problematic enough to make it "not worth the trouble" to the owners. Likely pitfalls:
- many players won't want to play overseas away from home, so those teams might be at a severe disadvantage
- there is no homegrown talent to cheer for, which also adds to the issue above
- you can't play any day games or you lose your primary viewing market (they are 6 hours ahead, so a noon or even 3pm game is in the early morning here, not to mention the West Coast)
- they can never be on Monday night or Thursday night football, so those teams don't get equal treatment. A weekday night game in London is an early weekday afternoon game here - you just lost a ton of your audience.
- travel is a problem, especially to and from the West Coast where we are now talking 9-11 hour flights
- the current games are well attended, but is that because of the novelty? If there are games in England every week, will the novelty wear off and attendance lag
In the end, I believe you will have to grow the sport through expanded viewing opportunities for Europeans and others around the world who are interested. You can't make it free, but I think it would be a sensible to do it as cheaply as possible. There is little risk in increasing the exposure and not reaping a ton of immediate profit from places where you see essentially near zero money from right now. As technology allows, via satellite and cable/internet, the league would be wise to grant cheap access to foreign markets to see where pockets of interest take hold and then exploit those markets in other ways from a profit perspective.
The only way football succeeds in non-US countries/regions is if they have their own leagues and then it will never succeed to the degree it has in the States. Soccer, baseball, hockey and basketball have become much more international because there are pockets of interest around the world and high levels of local participation, both amateur and professional. Football is not in the same position as those sports globally. You can't artificially plant the NFL in a foreign county. The best the "NFL" could do would be maybe some limited expansion into Canada and Mexico.
The other thing that hurts football is that it's not a sport where you can easily expose naive fans through high level exhibition type play. Soccer, baseball and basketball lend themselves much more easily to that. English Premier League teams for example are willing and able to play in other countries in exhibition games utilizing their front-line players. Can you imagine asking the Packers or Patriots to play a couple of exhibition games in the off-season in Germany to show off the sport, and to play your starters for a majority of the game!?!?