Here's a crazy idea: Burma avoids British colonization. Through a series of wars and diplomatic manoevering, China prevents Indochina from falling under French control, yet still loses influence.
Meanwhile, a Meiji Restoration begins in Siam. By the early 20th century, Siam is respected by Europeans as a power in its own right. Siam deftly stays neutral during WWI, but Burma and Cambodia gradually fall under the influence of European powers.
A combination of anti-European sentiment and economic troubles leads to the rise of a Thai fascist leader in the late 1920s. Said fascist leader proclaims the superiority of the Thai people, enthusiastically embraces Buddhism, and promises to liberate fellow Buddhist countries from European domination. Thai-backed nationalist movements appear in Burma and Cambodia. It becomes clear that Thailand seeks a Buddhist Empire.
When Hitler comes to power, the Thai fascist leader demands German assistance against Britain and France. Initially Hitler declares Buddhism to be compatible with the Aryan Race and agrees to aid Thailand. German-Thai relations turn sour before *WW2, but not before Thailand had built a German-trained army with a matching industrial base. To this day many believe the fascist leader foresaw WW2 years in advance and carefully used Hitler.
Anti-European rebellions, funded by Thailand, break out in Burma and Cambodia. With war raging in Europe and in China, the Thai army "comes to the assistance" of these rebels; Britain and France are in no position to challenge the Thai invasion. Japan attempts to recruit the *Buddhist Empire as an ally; this offer is kindly rebuffed and instead the US views the *Buddhist Empire as a potential ally against Japan. Japan invades Vietnam on schedule, leading to the US embargo on oil exports.
Japan makes its blitzkrieg on schedule. Japanese propaganda tries to incite Khmer and Burmese separatism, yet this backfires due to Japan's sheer brutality. Instead, the *Buddhist Empire gains legitimacy among its people. Vast amounts of American aid keep the Burma Road open to China throughout the war.
WW2 ends. The *Buddhist Empire is exhausted but united. With the Cold War looming, the US sees the proto-fascist *Buddhist Empire as an ally, and provides large quantities of military and economic aid.
Today, the *Buddhist Empire is a monarchy whose King is also head of the established Sangha. The average Burmese sees the average Khmer and Thai as a fellow countryman, albeit speaking a different language. There are free elections and free speech, though there are unspoken red lines that can't be crossed. The fascist leader who made the *Empire possible is seen similarly to Ataturk: he's generally revered for building a strong nation and his diplomatic skills, but it's also acknowledged he had his faults. At the UN, Muslim countries frequently introduce resolutions accusing it of Buddhist chauvinism and oppressing Muslims. The military, which declares itself the guarantor of the Buddhist character of the state, is often accused of lining its pockets and subverting elected politicians.
