Portland? Las Vegas? Honolulu? Maybe, but probably not, unless the NFL goes to 40
Here are five cities (the ones in light brown on the introductory map) that I see as long shots. Most of these cities would have to construct new stadiums, and many are in small markets.
Portland, Oregon: One of the largest markets without an NFL team; I see it as an also-ran because of its proximity to Seattle (it seems a debatable point whether the Northwest can support two teams) and also its propensity to adapt Multnomah/Civic/PG&E/Jeld-Wen Field rather than actually build a new stadium—a stadium that would be needed for an NFL team.
Las Vegas: A medium-sized market that has no pro sports teams; the largest market without pro sports. Why it's an also-ran? Because the Big Four is wary of putting a franchise in Vegas due to legalized gambling there. Also, a new (probably retractable-roof) stadium would be needed.
Honolulu: The site of the annual Pro Bowl, a team in Honolulu could potentially draw from Asian and Pacific markets (remember, football is big in Hawaii, and in places like Samoa). Obviously, not a hugely viable option because it's a smaller market, needs a replacement for Aloha Stadium and is a six-hour plane ride from any other franchise. But, I think there eventually will be a major pro franchise of some kind in Honolulu (especially if we end up with 40-team leagues instead of 30-team ones), and football would make the most sense because it involves weekly travel rather than daily travel.
Birmingham and Louisville: Kentucky and Alabama are in football-rich markets completely devoid of Big Four sports. However, they are small markets, close to other franchises, and would both need to build new stadiums. They seem like possible options if the NFL goes up to 40, but not really if it stays at 32 and moves some teams around.