I really hope the race picks countries which they either haven't visited yet or haven't visited in a long time.
Some countries which they haven't visited (and might appeal) include
Europe: Latvia, Belarus, Serbia, Malta, Israel (They love the TAR in Israel), Bosnia and Herzegovina
Americas:French Guiana, Guyana, Paraguay, Suriname and (A whole stack of Caribbean islands)-Greenland would be super cool too
Asia: Maldives, Nepal, Armenia, Azerbaijan (interesting if they visit), Cyprus, Georgia, Qatar, Bahrain, Uzbekistan
Just some food for thought