Tang was fully ethnically Chinese insofar as Chinese, in the northern states, was concerned. If you were to trace their ancestry, you would probably end up mixed between somewhere in the steppes and the Han homelands but if you were to use that distinction, then quite a lot of Chinese dynasties would very quickly end up foreign/invader/conqueror role.
Though speaking of people north of China, it would be pretty cool to have Jurchens/Manchu in Civ at some point. They've played a pretty big role throughout history and definitely qualify for even the oldest of Civ's requirements of a culture. They have an (endangered) language the leader could speak, they've created their own script. They've been a constant in Manchuria, have a looong history with Koreans (Jurchens - Koreans are kind of the same eternal duo as Monglians - Chinese).
Set up a good bunch of states: Goguryeo (probably a Koreanic state, but it definitely included and shaped these people), state of Balhae (another mixed Goguryo-Jurchen state, interacted with Tang, later Korean kingdoms as well as the Japanese of their time, all of whom wrote about the place), Jin dynasty (built the northern Great Wall, but Mongols had some unfinished business with the Jurchens right around this Genghis Khan fella'), Manchukuo (if you wanna count, keeping in mind there are still Manchukuoans living today) and of course the later Jin/Qing dynasty, which hopefully needs no introduction.
These guys also had a hand in the founding of the Joseon dynasty (you know, Sejong and stuff) as the dynasty's founder was, much like in the case of Tang, a Jurchen from the later days of Yuan and Goryeo (which itself was led by the Mongolian khan's cousins), and later when Qing came to power, all Joseon kings had to grow up with in the Jurchen (now Manchu) royal court, married at least one Jurchen princess and so on. As it happens, they were also the only people, until the US, to invade and occupy any portion of mainland Japan (yes, Jurchens did successfully occupy part of Kyuushu and had the imperial army drive them out).
They definitely have qualifications well beyond places like Cananda or one-hit wonders like the Huns as far as Civ is concerned and it would be nice, if highly unlikely, to see them represented as something other than some weird nobodies appearing out of thin air and taking over China, inventing the qipao and then getting by and large sinicised by the modern day. Plus points for at least once in history actually showing something related to Korean history that's not them being the tiny nobody between Japan and China (We got Silla and it was just repainted Joseon with a queen whose sole accomplishment is giving Tang the backdoor needed to succeed where Sui had failed).
But yeah, using Cixi as their leader would probably not work out well.
