In the early 1400s, China sent a huge fleet out on 7 expeditions -
en.wikipedia.org
[OTL] These voyages ended after the 7th, in 1433, largely due to a faction within the empire that opposed the emperor's extravagant spending.
[POD] The voyages continue.
I'm not very well-versed in this area, so this is all handwavium and speculation about what might be plausible:
- China develops a network of trade, tributary states, and competitors in SE Asia and the Indian Ocean.
- Further voyages "discover" distant lands to the east. To the south, they explore the coast of Australia and reach New Zealand.
- To the northeast, their "Columbus" finds his way across the Bering Strait and brings back tales of a vast land with rich fisheries and hunting, and natives who are "even less civilized than the Ainu".
- Exploration of this new land discovers an nearly endless coastline, reaching south into warmer climes. By 1492, there are Chinese settlements dotted down the coast to Central America.
- Meanwhile, the pressure of the Chinese expansion changes the course of south Asia. With more conflict, their advances in technology, militaries, etc. come closer to keeping pace with Europe.
- A number of small Indian kingdoms also form trade networks and establish outposts and settlements in East Africa. When Portugal rounds the Cape of Good Hope, they find a strong Asian presence.
So, by 1500, Asia is in a position to compete with Europe in both the Americas and Africa. There's a million maybes still to sort out, and some more centuries to write, but I think this gets them started in the right direction.