You need a UK division. Then somehow that winner gets a spot into the playoffs, two out of the 4 WC spots or something. They could never host a playoff game (too unfair). Imagine SD traveling to the UK, winning, then having to play next weekend in Denver? They give teams byes after making the trip and playing, so to stop that for the playoffs would be silly. The UK teams would also need a US "home base" for the playoffs, so their teams can stay, practice, have a top notch facility instead of constantly on the road in the playoffs. It would logistically be tough. Another way to do it, would be to have a minor league in the UK, but that's what college football lis these days and you aren't replacing that. Who knows.
I couldn't see them going into Mexico City. In the distant future probably. There are strong ties with the U.S. and Mexican Americans many of whom have ties to the home country and are big NFL fans. So there's a more natural fan base than Europe or Asia. Mexico is a developing country with huge potential. Natural resources, a population that is on the cusp of being great in terms of skills, education, a strong labor force. But it's always the same bugaboo with Mexico. Their government and a tremendous gap between the rich and the poor. Plus one other serious issue that is very substantial and should be the key reason the NFL doesn't go to Mexico. The power of the drug cartels who some say are even worse than ISIS. Until they are completely eradicated it's almost impossible to do business there. Even though they are currently the U.S. number 1 trade partner.
I could easily see them going into Mexico City, especially if Mexico pays well enough. Bringing in wealthy Americans, that's literally gold for drug traffickers. Here's what I see happening within the next 10 years: A Super Bowl in the UK 5-8 Regular Season games in the UK 1-2 Regular Season games played in Mexico or Canada or both Goodell and the owners have talked about wanting that for a few years now and slowly but surely they've been getting it and pretty successfully too.
