I was wondering are there many countries that people don’t visit? I’m fairly sure you can get a visa to visit North Korea and that Chinese tourists especially do so.
I would imagine Iraq might be too dangerous and also Somalia but with people into extreme everything else I’d say there’s a market for war-tourism.
If you find the subject interesting, I recommend P.J. O’Roark’s “Holidays in Hell”. He spent several years visiting political hot zones and armpit locations as a corrispondant for Rolling Stone Magazine, then wrote a pseudo “travel guide” about his experiences and observations.
Also, Robert Young Pelton is into this sort of stuff.
I believe I have a copy of that somewhere. I’ve only ever read the section on Belfast.
There is a bit of “tourism” in Somaliland which is the northern unrecognized part of the country. I myself have been on holiday in Yemen, Iran and D.R. Congo… and hoping to go to North Korea soon… which are not quite like going to Florida. 
Some older great travel books are “Danziger’s Travels” by Nick Danziger (much time in Afghanistan in 1984 during the Soviet occupation) and “Investment Biker” by investment guru Jim Rogers about his round the world motorcycle trip in the late 80s or so.