Personally, I don’t want more Asian civilizations due to how they are designed.
And as for my thoughts on the Burmese, they were not big enough to be ignored like the Indians, nor had they managed to maintain their sovereignty independently like the Siamese. In my preference, I’d have them introduced as mercenary units.
This is actually a quite unimportant and low priority reason.
It’s not necessary just because AoE1 or AoE2 introduced them, so AoE3 also has to introduce them as a complete civilization.
If you still think that being a mercenary or native minor civilization brings too little new content for the small Asian countries like Burmese, Vietnamese, Koreans, etc. , you can consider this new idea about the King of the Hill. They will become a special minor civilization “Landmark” with richer contents and more importantance to the game. This avoids the worry of having too many civilizations because a large number of small Asian naitons have become complete civilizations.