Disclaimer; my knowledge of the area's history is sketchy, so it may have happened already, or impossible per see...
I know vague oulines around. The area roughly southeast of china to Malaysia/Indonesia, Burma to Vietnam... I know the Chams some, who used to be powefull and mighty, and muslims later. The fight of Siam and Khmers, now Thailand and Cambodia.
Was there openings in history, POD, opportunities, for somes of those powers to go beyond and grow in territories and might, to unite parts, if not nearly a whole or most of the area at all even? An Indochinese Empire, BEFORE the colonial era of the occidentals, to add complexity (if possible).
Was there factors for or against it, and reasons to do so, or not? Is that possible, probable?