I must say that it is nice to get a souvenir from each country.... however there is also a discussion about what constitutes a country. Some more remote islands such as Easter Island are part of Chile, yet should have a souvenir as they are a separate unique place.
I would suggest that Geocaching should look closely at the Amateur Radio Hobby as having already sorted this one out, as hams also have "bragging rights" for countries... in their case, talking to another operator in a country. The Amateur Radio Relay League (ARRL) has a "DXCC" award, in which operators talking to a hundred countries or more get progressive certificates.
The point here, is they have an approved countries list, which includes many small remote places, that could be used and presently has 339 entities.
The most recent list is posted at http://www.arrl.org/files/file/DXCC/2016%20DXCC%20Current.pdf
If this seems too extreme, the UN also defines a countries list with just under 200 countries. Since countries come and go over time if a souvenir is to be developed for each country, then those who want to target more souvenirs need some guidelines as to what countries are "approved".
Just a thought... my Geocaching stats say I have found a cache in 37 counties, but no souvenirs yet for some of them .
Darwould.