What really happened to China if Timur (Tamerlane) never died?

Nobody could be able to precisely describe what happened if he invades China. But most likely possible that he would finally conquer the whole Ming Empire.

Reasons:

1. Emir Timur (Tamerlane) was an undefeated warlord in over 45 years (1360–1405) of his military career, despite facing outnumbered enemies, harsh terrain, and formidable enemies like Ottoman Turks, Egyptian Mumlukes, Tartar Golden Horde, and Delhi Sultanate. 

2. As there is no doubt, he is a military genius and brilliant strategist; he overcame every odd and remained unchecked by any military of his time. He is the greatest conqueror of all time. He conquered the entire landmass from Mongolia in the east to Turkey in the west and from Russia in the north to India in the south.

3. His hordes overrun all the parts of the former Mongol Empire and united all Mongols/Tartars under his banner and prepared with 200,000 soldiers to conquer the Ming Empire. However, Ming Chinese too assembled an army of nearly a million soldiers to counter Timur. Besides, Ming Chinese also fortified and increase the size of the Great Wall of China to protect from his invasion. 

4. After the death of Timur, his general and protégé Buyensher (famously known as Oljei Temur Khan) defeated the Ming army (1409), even in that time Tamerlane wasn’t there but still Northern Yuan Dynasty capable enough to inflict a defeat on the punitive expedition of the Chinese army, particularly 44 years later after Timur's death, the Oirat leader Esen Taishi defeated the 500,000 soldiers of strong Chinese army led by Ming Emperor Zhengtong at the Battle of Temu (1449) with just 20,000 cavalry and even captured the Chinese emperor in battle, if suppose Timur with his incredible personality could invade China together with his all Mongols (Yuan, Chagatai, Il-Khanate, and Golden Horde), which were previously subdued by Timur, then obviously odds were in his favor. 

However, there is no doubt that the encounter between these two giants would bring a titanic struggle. But I think it was most likely plausible that Tamerlane would finally emerge as victorious. Even the invasion of China never happened because he died from fever at Otrar, in Kazakhstan but he left his fear on Chinese, even after his death because of this sheer fact, his son and successor Shah Rukh gained nominal suzerainty over China (even India as well).