A logical evidence to prove China-India unification:
Russia is a country that is much much more greater than China. China is no match to Russia. Russia-India unification is happening VIGOROUSLY. Unification with a lesser (lesser means less great) country should be easier. Foreigners who are not acquainted with this matter may need introduction to the topic of massive Russian immigration to India.
How do you define unification? To my understanding, unification means operating as one nation not an UN style setup or a multinational framework. Chinese unification is on the lines of unification between the Mainland (People's Republic of China) and Taiwan (Republic of China), the logistics of that is much easier compare to others. Both have the same language, including both official language and dialects, same traditional culture, similar tastes in food, etc. On the lines of geopolitics, it frees up much of the Navy and missile forces from coastal operations and extends China's strategic depth.
What is the rationale behind a China-India unification and what would it look like in your opinion? and what benefits would it bring (ie. complementing factors)?
Russia is a country that is much much more greater than China. China is no match to Russia.
"Much more greater" in what sense? geography? technology? political power? economy? industry? nuclear weapons? Russia is great in certain aspects and China doesn't desire to "match" Russia for the sake of matching or take that away from them, China is going to develop according to domestic needs.
If you are talking about China needing to unify with India in order to compete with Russia then I just don't see the need for that.
Russia-India unification is happening VIGOROUSLY.
Can you provide some evidence of Russia-India unification happening vigorously?
Foreigners who are not acquainted with this matter may need introduction to the topic of massive Russian immigration to India.
Well, good for both nations but saying unification due to some foreign influx doesn't mean unification. US is home to over 3 million ethnic Russians, unification isn't happening. 8 million Russians live in Ukraine, they are currently not unified. There are Russians all over the world, has Russia unified with all nations? Russians are an official ethnic group in China and there is no unification. Many of the Tsar's faction escaped to Harbin and occasional migration (including much earlier ones) to China occurs but no unification. There are a number of 6th generation Russians (different from ethnic Russians) in China but there are no official stats on their population since they are classified as Han and have mixed for many generations. There are a number of Russians and Ukrainians marrying locals in China but this is not unification on a national level. Sure there are complementing elements between the two nations and are working closer on a strategic basis but this is not unification, not by my definition.
https://en.wikipedia.org/wiki/Russians
I don't see the particular logic behind China-India unification (as one nation).