How about... The Chinese Civil War stretches a few more years, and the Nationalists finally defeat the Communists totally. A Communist revolt in American-occupied Japan scares the USA, and many American soldiers are killed and tortured, what with the bad blood with America going around in Japan.
The Americans get sneaky and turn the occupation over to China. After a second big revolt, China announces outright annexation.
In this TL's version of the Korean War, South Korea annexes North Korea with heavy Chinese assistance, and forms the Korean Republic. China puts massive effort to help the Koreans build infrastructure to create a unified state. After a few years, the Koreans rely on Chinese help so much that they agree to becoming part of a EU-type organization founded by China.
Thailand, Burma, Nepal, Bhutan, Tibet, India, and the Indochinese states (including a united Vietnam) all join the organization, seeing China as a bulwark against Soviet Incursion.
By the Seventies, The Phillipines, Malaysia, Brunei, Indonesia, Pakistan, Bangladesh, and East Timor have all joined this organization after the Soviets invaded Afghanistan and set up an Afghan SSR. The organization federalizes for the next half-decade, and expands to include a failing Persia, Iraq, Saudi Arabia, Yemen, Oman, Qatar, Bahrain, the UAE, Syria, Lebenon, Jordan, and Israel.
By the late Eighties, the Asian Union has almost fully federalized, and a President of Asia is elected. Although China is seen as the leader of the organization, the leader of China decides that if the President of Asia is not Chinese, then people will not be so quick to see the Asian Union as a Chinese machination.
The Soviet Union collapses on time, and the varies Central Asian and Caucasian states join the AU. Mongolia, free from Soviet influence, also joins.
Turkey later joins as well. By the twenty-first century the only country in Asia not to join the AU is Russia. Russia is decaying socially and economically, and in 2005, it's split into Eastern and Western Russia, which join the AU and TL's analog of the EU, respectively.
Yeah, I know, not plausible at all, but it works.