You cant "focus on diversity", identify and culture is not manufactured. You are viewing history through purely a modern prism. That's an incorrect approach.
A cultural identity in all its manifestations of language, food, beliefs etc... is developed by a collective consciousness, it does not grow by design.
More then 90% of China is of Han ethnicity. Whereas the largest ethnic group in South Asia are the Punjabi (Pakistan 120 million, India 40 million) and Bengali (Bangladesh 160 million, India 80 million).
Population of South Asia is about 1.8 billion. So Punjabis are about 9% and Bengalis about 13% of South Asia. Both these groups are totally different to one another in every meaningful way.
A historical India is a moot point, because it has no basis in reality. just as a historical European nationhood is a logical fallacy. South Asians share cultural similarities among other things, just like the Europeans or an African but there are many distinct actual historical nations.
The Chinese decided to adopt a unified culture, of which language is a primary part, by recognising the need for it. If it was so simple to create an identity, the Jews would be no more, they have faced immense levels of persecution at the hands of the Europeans throughout history, but identity cannot be forced, if the recipient is not willing to adopt it.
So, China is a nation because you guys over time adopted a Chinese identity. South Asians are not a nation because there was no identifiable Indian identity to hold on to. A Punjabi, Marathi, Bengali, Tamil and so many other groups created their own identities because that was what they wanted. Even today, India does not have a national language because the south region of India refuses to accept Hindi. They have two official languages, Hindi and English and many scheduled languages, but no NATIONAL language. So, even today an Indian nationhood is a work in progress, how can there be a historical Indian-ness, when even today it is still being moulded.
There are so many facts negating the historical Indian nationhood, but I've tried to present the essential ones, I hope you will find it helpful.
@Pan-Islamic-Pakistan @Mangus Ortus Novem @masterchief_mirza @SecularNationalist