I don’t think I’ve ever fully written a completely unique country before. Usually my stories are set in something akin to our world and countries or in already developed fantasy countries like Equestria. I like world building, just not enough to go all the way and create an entirely new country with its own culture, language, customs, all that. I much prefer writing characters over world building.
Now if we’re talking something like making a whole new country in the current state of the world, I’ve always been attracted to the idea of the Great Lake states and provinces becoming their own full fledged country. Kaiserreich, a mod for HOI4 where Germany won the First World War, has this to an extent. As the Second American Civil War in that mod has the entirety of the Great Lakes declaring independence and becoming their own country.