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.