Good point on the international expansion, and I think it's reasonable to assume that they'll push out some domestic expansion over the next 40 years, particularly in Mexico City. Expansion in Europe is possible, but will meet some resistance on the whole "road game fatigue" scenario. It'll happen in time, though.
However, we've seen on the domestic front that an NFL franchise can survive and thrive as long as there's a pool of about 9 million fans per team. You can argue the pros and cons of a particular site, but NFL expansion has been successful at that rate over the past 80 years, and I see no reason that it won't continue.
Now, one might argue that Carolina and Jacksonville are the first cracks in that model. I'm not sure if they've achieved national visibility to the same extent that earlier franchises did, and it seems like Jacksonville in particular has battled to gain a permanent foothold against an entrenched in-state team (or two). Carolina I'm not so sure about. Maybe they're stable, and they were definitely a perfect spot in which to place a team.
In the future, new teams may not end up being nationally known teams as easily as long-time teams like the Bears or the Cowboys or the Steelers. Success will play a big role in that, of course, but I think the real future for NFL teams is niche marketing. As we expand to 40 teams and upward, the model may be less to keep fans aware and knowledgeable about all NFL teams, and more toward building an ardent and loyal fan base drawn from about 10 million people in the region. It'll be more about building through niche marketing than leaguewide marketing.