What if the Formosan aboriginal peoples formed more prominent kingdoms on the island? There was the Kingdom of Middag, but that was a later creation and had the issues of Dutch/Spanish rivalry fighting over the island in addition the Chinese at the Kingdom of Tungning and Japanese attempts to claim the island as well, not to mention piracy. I'm thinking of an earlier PoD would be good for establishing some early polities, the 15th century perhaps? And in terms of number, minimum of 4, centered around Sirayan/Paiwan lands to the south, Amis lands to the east, Atayal to the northeast, and Hoanya/Taokas to the west, maybe splitting those with slashes in them into their own?
How would the presence of notable kingdoms (or chiefdoms) on the island affect its population? How about trade with China, Japan, and the Philippines to the south? And what of later colonization attempts?