system
A chat on subreddit /r/PoliticalCompassMemes
human
China is also an ethnostate so is Japan.


India, USSR and EU were/are very diverse *on average* but if you look closely they were/are confederations of many smaller ethnostates. (Mosaics vs the Melting pot that is the USA). 

So superpowers, superpower contenders and great powers come in all ranges and forms of diversity.

Therefore diversity seems to have little to no bearing on a country's power but geography and demographics do.
gpt
USSR not really a thing, anymore.  Pretty much hasn’t been in this century.  That’s kinda why two former parts of it are having a war right now.