Why not Mexico? Well, Mexico puts on an undeniably great World Cup. If that and regional rotation were the main criteria, I'd say they had a great shot. But since they've hosted twice already in the past 50 years, it doesn't seem right to go back yet. Here's the cumulative list of hosts:
Mexico: 2 (2 post-war)
Germany: 2 (2 post-war)
Brazil: 2 (2 post-war)
France: 2 (1 post-war)
Italy: 2 (1 post-war)
USA: 1 (1 post-war)
Spain: 1 (1 post-war)
Sweden: 1 (1 post-war)
Switzerland: 1 (1 post-war)
England: 1 (1 post-war)
Argentina: 1 (1 post-war)
Chile: 1 (1 post-war)
South Africa: 1 (1 post-war)
Japan: .5 (.5 post-war)
South Korea: .5 (.5 post-war)
Uruguay: 1 (0 post-war)
Mexico's only peer as far as frequency is Germany, and even then you could say that parts of Germany have only hosted once. Brazil has two now, but their first was in 1950, generations ago. Mexico has had two since 1970. Now, I could see scenarios where joint hosts include Mexico: either a Can/US/Mex Cup, or a Mex/Latin America/Caribbean Cup. But I'm not sure the entire country, solo, would be the right choice to host again so soon.
If 2022 is in play, I'd favor Turkey (for geopolitical reasons) or Australia (though they're CAF, they were OFC until recently and are, in practicality, separate world region deserving of being included in the rotation). Depending on how that worked out, England and the USA (or a North American joint bid) should split 2026 and 2030 in some order.
In general, the concept of joint or regional bids - which FIFA has traditionally disliked - open up many more exciting (and politically palatable) possibilities for hosting. I hope that's an idea that's embraced.