Here’re my ideas about potential DLCs for East and SE Asian civs, and the possible unique units (haven’t really think about their civ bonuses or their unique techs apart from the Chams and the Dians/Zangkes)
First a major DLC called Warriors of the Eastern Steppe, featuring Gokturks, Jurchens, and Tanguts.
Gokturks: their UU would be a heavy melee cavalry called Fuli or Bori, and its specialty is that it can continue fighting for 3 seconds even after it has lost all of its HP.
Jurchens: their UU would be the Tiefutu, a slow and short-ranged heavy cavalry archer with high pierce and melee armors, and its attacks ignore enemy pierce armor.
Tanguts: their UU would be the Camel Slinger, basically a camel rider armed with a slingshot and is good against infantry, can be seen as a tankier and faster version of the Inca Slinger.
New regional UU: Fire Lancer, has the same attack and range as the Fire Ship and is good against standard buildings and siege weapons. It’s trained at the Siege Workshop and is available to Chinese, Jurchens, Koreans, Mongols, Tanguts, and Vietnamese.
Gokturks don’t have access to the Fire Lancer, but they share the Steppe Lancer with Cumans, Mongols, Tanguts, and Tartars. Tanguts and Mongols are the only 2 civs that have access to both regional units.
Regarding the architectural style, the Gokturks, Mongols and Tanguts would share a new Inner Asian style that is loosely based on the Tibetan style. The Chinese, Jurchens, Koreans, and Vietnamese would share a new Pan East Asian style based on the Chinese style. And the Japanese would continue using the current East Asian style (which is based on the Japanese style).
Secondly, a minor DLC called “Peripheries of the Sinosphere”, featuring only 2 civs, Dians/Zangkes and Ryukyuans.
Dians/Zangkes: for their civ proposal, please check this thread that I made a while ago, I recently made some changes and updates to it (initially I called them Nanzhao): Civ Proposal: Nanzhao - Age of Empires II: DE / II - Discussion - Age of Empires Forum
Ryukyuans: their UU would be the Three-Barrel Hand Cannoneer, which is like a Hand Cannoneer version of the Chu Ko Nu, good against infantry and siege. This type of weapon came from Ming China, but through trade the Ryukyuans obtained some of them and equipped some of their Ashigarus with this gun.
Dians/Zangkes would also share the Fire Lancer unit with other continental East Asian civs, and they share the new Pan East Asian architectural style with them as well.
Ryukyuans, of course, would share the Japanese architectural style with the Japanese civ.
New regional UU specifically for Japanese and Ryukyuans: Ceramic Grenadier, which is again a siege infantry trained at the Siege Workshop, good against standard buildings and defenses.
Thirdly, another minor DLC called “Tales of the Southern Kingdoms”, featuring Chams and Siamese.
Chams: please check this thread, I recently made an update to it as well The Chams - new civ concept - Age of Empires II: DE / II - Discussion - Age of Empires Forum
Siamese: their UU would be an elephant carrying a small cannon on its back, and shoots out the same projectile as Hand Cannoneers and Janissary. But unlike those two units it has a blast radius and has a bonus attack against standard buildings and defenses. Their secondary UU is an unique upgrade to the Elite Battle Elephant called the Imperial Battle Elephant.