'AHC: Pacific Carthage'.
As in, a fearsome mercantile power in the Asia-Pacific that establishes a large trade empire. Japan's the obvious candidate, but as I understand it, they never developed much of a commercial streak until the US came in and Westernized it after World War II, so...
In this context, I fear that Japan makes for a poor candidate. I know that I've proliferated the notion that Japan played the part of "Carthage" to America's "Rome" in World War II (bt let's not get into that), yet that comparison relies on the geopolitical role involved. It does not suggest that Japanese culture is anything like Carthaginian culture.
You'd have to look in South-East Asia, for sure. Traditionally, Southern China was more oriented towards the sea. (Most of the crewmen on Zheng He's voyages were, in fact, Cantonese.) This goes way back. You may also note that Chu was the most laissez-faire of the warring states, and that these attitudes were only consolidated when they absorbed the Yue in the far South. (Yue is a cognate of Viet, by the way-- directly related to the people of Vietnam.) Qin rule was obviously
not laissez-faire, but the Han dynasty (which was from Chu) again embraced the hands-off mentality familiar to Taoism of the period. (However, the drive to strengthen the imperial identity of China forbade trade ventures further afield; consolidation was the greater priority.)
For this reason, I find one excellent POD for an East Asia trade empire in a different conclusion of the warring states period. Specifically: one where the unification fails. This relies on Qin being destroyed, and Chu consolidating the South. Subsequently or concurrently, one of the Northern rivals must unite the North, with a long-term stalemate being the result. The resulting realm of Chu could easily expand further South. First, by dedicated all its efforts into absorbing all of Vietnam. But thereafter, greater parts of Indo-China, as well as Taiwan, the Philippines, Indonesia... possibly even Oceania. This would by definition by an ocean-facing trade empire, able to do
excellent business with the (equally trade-oriented!) mercantile realms of South India.
(You'll note that this is the scenario I propose for
A Thousand Sunlit Years.)
If you seek a later POD, anything that ends up fracturing China is a potential springboard. The above POD is excellent because it aborts the influences of Legalism and Confucianism to a great degree. But later PODs can definitely work. There were also various periods of Chinese mercantile diaspora in OTL. After all, when Zheng He reached such places as Malacca, he found a thriving Chinese community there! These phases were, as far as I can tell, always instigated when China was fractured and/or invaded from the North. This typically created periods wherein the South of China was independent and self-directed, and also facing bitter enemies to the North. Every time, they reacted by turning towards the sea and instigating trade. This tells me that if you want a Chinese trade empire, South China is to be the cradle for it.
Alternative options include OTL's mercantile empires in Indonesia. Although, as we've discussed in conversation, these were typically somewhat 'brittle', and often one really bad ruler away from getting screwed over by the competition. Said competition was often the aformentioned merchant-princes of Tamilakam. It didn't help the Indonesians that they're much closer to Southern India than a Southern Chinese trade empire would be. (The outcome is that their heartland could be attacked directly.)
