So a Portuguese-Korean alliance in the 16th century?
Or a Spanish-Korean alliance. The Spanish can be butterflied away from the Philippines if Magellan's expedition fails to cross the Pacific but it's ASB to assume that the Spanish can be butterflied away from East Asia forever. The Spanish do have the silver that the Chinese need. And the Portuguese do not have it. But where can they meet?
Perhaps the Spanish first get interested in New Guinea. They will need to cross the Pacific somewhat earleir than Mendana, De Qujros and Torres's 1598 voyage from Peru, but once Peru is conquered, that much is likely. And the Pacific will not seem as daunting if the Spanish expedition leaves from a place like Arica and discovers the Marqueasas Islands and/or Tahiti and then other islands---with an expedition from Mexico discovering Hawaii before discovering the Micronesian islands.
At that point, with the Philippines sewed up by Portugal (and very likely, nutmeg and pepper trees transplanted to the Philippines already) , the Spanish are going to be looking farther north as well as south.
New Guinea can be explored by a would be conquistador, and if such a conquistador lands at Lae Gulf on the north coast, his expedition will very likely reach the New Guinea "tierra templada" highlands and the farmers who think the Spanish are ghosts, making New Guinea a potential base even before gold is discovered there. And gold will be discovered in New Guinea.
From New Guinea (and Micronesia to it's north, it is not a difficult sail northwest to the Ryukyu Islands. If the Spanish can conquer the Ryukyus before Satsuma conquers it in 1605, the Spanish have one of their connections to the Chinese silver market via the Ryukyu tribute mission yearly to Nanjing.
And initially, Japan may also welcome Spain as well. Unless Spain finds a better welcome in Korea.
Korea is another place where Spanish silver (as well as Hawaiian and New Guinean sandalwood) may open many doors. Especially if Spain can get in before the Japanese invade Korea. Because when the Japanese do invade Korea, the Koreans will want Spanish guns and maybe even military advisors. And be willing to tolerate Christian missionaries in the Korean countryside to get both.
And if Koreans become as enthusiastic about Christianity in the 16th and 17th Century ITTL as they are in the 19th IOTL due to their belief in their one god Hanumim, Spain may have the fifth column within Korea that it needs to actually take over the place by say, 1650 should the Yi attempt to crack down on Christianity. Not to mention be given some new ideas on naval tactics such as low to the water iron clad "turtle ships" from Korean Admiral Yi Song ye that may have unpredictable butterflies in Europe. And quite likely lead to early Spanish settlement of eastern Australia, which is undisputably within the Spanish sphere of settlement. As is Korea and Japan.
And the Dutch?
The Dutch may find themselves held to Java, southern Sumatra, the Lesser Sunda Islands and Western and Northern Australia. Maybe Southern Borneo. With the English having to work their way into East Asia edgewise. Possibly via the Andaman Islands and a concession from Thailand to build a canal across the Kra Isthmus.