Germany (finishing previous tentative list, I expect a couple of the newly nominated sites to be inscribed, but little else)
I don't quite agree regarding Germany. Of course most worthy sites have already been nominated but some of the proposed sites (Darmstadt, Neuschwanstein, Caves of Ice Age art, Baden Baden) are world class. Moreover, we mentioned some nominations that could be successful in the future (Tempelhof, Olympia in Munich). AEG factory (a former TWHS) and some vernacular architecture (Rundlingdörfer or Altes Land, both of which contended for the last tentative list) could also be successful.
I completely agree regarding the other countries. I would also add: Albania, Bulgaria, Romania, Hungary, Slovakia, Czechia, Austria, Malta, Cyprus, Ukraine and Belorussia as countries with not much potential for further nominations.
Portugal - (updated tentative list coming soon.)
Poland - (tentative list update reportedly in the works.)
I think these countries as well as former Yugoslavia are also approaching their full potential of successful nominations.
Spain, Italy, France and Greece can still offer quite a lot.
I would also add Russia, and to a lesser extent Iceland, as a potential source of multiple future nominations.
What other countries in the world would we consider as approaching their nomination potential? Some candidates could probably be Israel (!), Canada, Argentina, Bolivia, Costa Rica, Uruguay, Jordan, Malaysia, Kyrgyzstan, Turkmenistan, Sri Lanka and probably many more.