As is fairly well-known, the Zheng family, a dynasty of armed merchants based in southeastern China, conquered the Dutch colony of Taiwan in 1662 and created an extremely centralized kingdom (the so-called
Kingdom of Dongning/Tungning). They also planned to conquer the Spanish Philippines on two separate occasions, and every European observer actually believed that the Zheng could have easily conquered the colony in either case if random circumstances hadn't made them abandon the campaign (in 1663, it was the sudden death of the merchant king Koxinga; in 1674, it was the Three Feudatories' revolt which convinced the Zheng family that they could invade China again instead of fighting the Spaniards). What is less known is that the Zheng helped the king of Cambodia kick out Europeans from his court and allied with Muslims such as the Indonesian sultan of Banten, even transporting Muslim merchants from India all the way to Japan.
The Zheng family was finally destroyed in 1683 by the Qing empire, which thereafter showed no interest whatsoever in expansion overseas. But this didn't have to happen. In 1677, as the War of the Three Feudatories raged on, the Qing government actually offered the Zheng family total independence under the condition that they retreat entirely to Taiwan and leave the entire Chinese mainland for the Qing:
How can two tiny islands match the vastness of the Four Seas? How can a crowd of rabble compete with the majesty of ironclad cavalry? How can salt and fish of the sea compare to tribute from fields across China?
[...]
[So] why do you not lay down your weapons and rest your men, bring all your carriages and armor to protect Taiwan, and count yourself among the overseas guest ministers [i.e. tributary kingdoms of China]? As for receiving ranks and titles, it is only at your wish. If you do not wish to do so, you do not have to receive them [i.e. the Zheng do not have to be Qing vassals if they do not wish]. How can our Dynasty regret granting a remote and impoverished corner of the seas for your rulers and ministers to preserve their names and rituals?
The Zheng refused and the Qing conquest came six years later.
What if things had gone differently?