Then by your own definition of "diverse", just about every people is already diverse, and your argument is moot. Every country in the 21st century is the result of centuries of conquests, domination, incorporation, or fights for independence.

As for Asia, that is precisely my point. Different empires existed in Asia at various times, with dominated people fighting for their autonomy from the conquerors, more or less successfully. 

But an empire by definition is one country or people dominating the others, whether it is Rome or Britain or China or the Aztecs, etc. It does that because it has a strong unifying culture it imposes on others, or that others admire and want to be a part of: Roman, British, Chinese, Aztec, etc. 

All of which does nothing to support your argument about "diversity".