We're getting to the point where new countries every season is not going to be a given. Many of the remaining countries that haven't been visited pose logistical or safety issues, and the show did more than the recent usual new countries these last two seasons.
As it was, we came close to losing track of them this season as we did in TAR 19 filming, but we were helped by return visits to Shanghai and to Indonesia, and TAR is seen in enough places that there's no way the show could film in Moscow, Amsterdam and New York City without being spotted.
I think there are many countries which are possible on all continents. Here is a list constructed to the best of my ability to remember which countries teams have visited and which they have not:
Continent of Antarctica
AFRICA (36 countries not visited yet)
Algeria
Angola
Benin
Burundi
Cameroon
Cape Verde
Central African Republic
Chad
Comoros
Congo
Congo, Democratic Republic of
Djibouti
Equatorial Guinea
Eritrea
Gabon
Gambia
Guinea
Guinea-Bissau
Ivory Coast
Lesotho
Liberia
Libya
Mali
Mauritania
Niger
Nigeria
Rwanda
Sao Tome and Principe
Sierra Leone
Somalia
South Sudan
Sudan
Swaziland
Togo
Tunisia
Uganda
Zimbabwe
ASIA (25 countries not visited yet)
Afghanistan
Bahrain
Bhutan
Brunei
Burma (Myanmar)
East Timor
Iran
Iraq
Israel
Jordan
Korea, North
Kuwait
Kyrgyzstan
Laos
Lebanon
Maldives
Nepal
Pakistan
Qatar
Saudi Arabia
Syria
Tajikistan
Turkmenistan
Uzbekistan
Yemen
EUROPE (19 countries not visited yet)
Albania
Andorra
Armenia
Belarus
Bosnia and Herzegovina
Bulgaria
Cyprus
Georgia
Latvia
Luxembourg
Macedonia
Malta
Moldova
Monaco
Montenegro
San Marino
Serbia
Slovakia
Vatican City
N. AMERICA (17 countries not visited yet)
Antigua and Barbuda
Bahamas
Barbados
Belize
Cuba
Dominica
Dominican Republic
El Salvador
Grenada
Guatemala
Haiti
Honduras
Nicaragua
Saint Kitts and Nevis
Saint Lucia
Saint Vincent and the Grenadines
Trinidad and Tobago
S. AMERICA (4 countries not visited yet)
Colombia
Guyana
Suriname
. Venezuela
OCEANIA (12 countries not visited yet)
Fiji
Kiribati
Marshall Islands
Micronesia
Nauru
Palau
Papua New Guinea
Samoa
Solomon Islands
Tonga
Tuvalu
Vanuatu
That gives me a total of 113 countries not visited yet. There is a lot of opportunity to have new countries in each and every Amazing Race, even with the necessary limitations due to safety risks.