I apologize if I seem to be nitpicking, but when you say nation, you're referring to countries, the political entity and not nation as in ethnicity right? If that's the case, the first one that comes to mind is the state of Manchukuo (1932-1945) which was located in what is today Northeast China. (Map: http://go.hrw.com/venus_images/0531MC23.gif)

If you meant as in nationalities that were extinguished so to speak, it wouldn't fit the bill. Though it was ostensibly run by the Manchus with the last Qing emperor as head of state, Manchukuo wasn't even a majority 'Manchu' state let alone even a significant minority.

Nor was it even the multi-ethnic kingdom touted by the Japanese as a country with "Five Races Under One." If you look at this picture (http://www.imperialjapanmedalsandbadges.com/sitebuilder/images/manchukuo-256x191.jpg) , you can see Manchukuo's flag, each color representing a 'race.' While the Manchus, Koreans, Mongols, Japanese and Han Chinese were the respective 'races'/minzoku , the Han Chinese alone were a majority 95%+.