In 1911, the Mongols elected Bogd Khan as the Khan of the Mongols. The cities in Inner Mongolia and Xinjiang with Mongol majorities also proclaimed loyalty to the Bogd Khan. In 1919 IOTL, Chinese troops occupied Mongolia.
POD: Mongolian forces drive the Chinese out in 1919 and maintain their independence. Therefore, the Mongolian People's Party is not founded and the Bogd Khan remains in power. When Russia collapses into civil war, the Mongols use the situation to their advantage, occupying Tuva and Buryatia. When the warlords take power in China, the Mongol Khanate wages war against the Xibei San Ma warlords and the East Turkestani Republics.
If Japan invades China, the Mongols ally with Japan and seize more Chinese territory, becoming even more powerful and strong enough to resist Chinese, Russian, and Japanese incursions.