One way not to organize society is to bring together different groups of people of different ancestral origins based on meritocratic traits which you choose and expect them to gel together based on an artificially created "national identity".
Because of how humans feel and think about identity. It's probably impossible to make a person think of themselves flatly and absolutely as "a citizen of x country" for example. This seems part of the inherent human 'software' and cannot be changed.
People feel and think first of themselves as an individual, then maybe in terms of their nuclear family, and then their extended family and friends, and then maybe in terms of their ethnic group... continuously expanding outwards in that way.
Attempting to bypass or jettison that natural gradation doesn't make much sense. It's contradictory to fundamental human psychology. Maybe even causes psychological issues in humans.
The far better way to organize society is by what is natural, in the way humans feel identity.
Some people think this is unnecessary and that you can reshape anything you want anyhow you want it to be. But that's not true. The natural world isn't infinitely malleable. There are limits to human control over the natural. And then, what is to be gained from a flatness of identity exactly?
Different societies with distinct identities necessarily have their own distinct cultures. If you did bring together people from different cultures, it only makes sense that differences in their cultures would result in clashes between them. So you do not want multiple, non-interoperable cultures in the same broader society.
Why you do not want multiple, non-interoperable cultures in the same society:
(i) Homogenization of culture is fundamental to a functional society. Culture, material and non-material, in the societal sense, really is just about standardization of traits, so that people are as interoperable and able to coordinate as much as is possible. Why, ever, would you want to interfere with that by having competing cultures in the same society?
(ii) Humans are inherently tribal. Differences between human groups with differences in identity necessarily means competition for resources, power and everything else. The in-group vs out-group thingy is probably fundamental to human psychology and thence human societies. Why would you want to complicate your society in that way?
"Yeahh. But at scale, across large populations across large areas of land, it's impossible for the cultures of people in a very large society like a modern country to be exactly the same."
True, but differences in culture aren't the same. Cultures are more related and inter-operable than one another. Especially if they share recent ancestry and influences.
So it shouldn't be surprising for example that societies which are close together are more inter-operable than ones which are far apart.
Some people may try to invalidate that by claiming that even culturally similar societies do have differences that result in their going to war against one another. While that is true, those wars are usually about resources and not non-inter-operable cultural differences. Nonetheless, there are lots of factors on which societies can differ and culture is only one of them. Societies with differences in culture have lots of things on which they differ. And culture.
That's clearly worse.
Societies with similarity in culture may go to war against each other, but we at least know one thing which they have in common. That's one factor which isn't an unknown quantity. It means less complexity.
Some people may bring up the example of what is currently humanity's most successful society 'The United States of America' and say "see? Multiculturalism works!"
The contemporary US is no evidence of anything. The multiple cultures for a long time were inter-operable western European cultures. And certainly, it is becoming increasingly multi-cultural and multi-racial, but in a time during which there has been a clearly dominant group.
There has never been any real competition for resources and power by non-inter-operable cultures. It's very clear who is in charge. Apparent problems are likely to begin to show up more clearly as demographic numbers change (maybe they are already starting to).
I have been using "multicultural" and "multiculturalism" in this piece, and that might make you wonder about how race fits in. It turns out that the piece isn't about culture, but "group identity" in general. And race is an even more fundamental layer of identity than ethnicity (and its related culture): it's inherent in people, genetically transmitted, and can never be changed.
If you understand the identity reason not to put people of the different non-inter-operable cultures together in a society, you especially do not want to do it if they are of different races.
In designing a society, you ideally want to increase chances of continuous societal prosperity for as long as is possible. And that means trying to reduce complexity as much as is possible almost indefinitely. Which means not designing things in a way that might cause predictable inter-cultural/inter-racial problems, no matter far into the future they might be.