I know I've made a few tentative suggestions for civs before, but I have a more concrete idea in mind that I think fits this mod (and particularly its emphasis on the Classical and early Medieval periods) well! The
Xiongnu -- also representing the
Xianbei and
Rouran, who can also respawn (between 900-1150) as the
Khitans/Great Liao and Kara-Khitai.
I think this would do a number of things. It ensures a more populated Tarim, Manchuria, and Mongolia to a degree that greater lines up with history--and a Xiongnu/Xianbei (and the Khitan revival!) would compliment the more dynamic China in this mod. Besides China, it also gives the Turks someone else to play off of in their early game, as would be accurate since the First Turkic Khaganate sprang from the Rouran. The potential threat of the Kara-Khitai also gets around the Turkish safety from barbarians while helping to push the transition to the Seljuks more naturally. It's actually a current marathon game as the Turks in your mod that really solidified this idea, as my efforts to loosely trace history led me to realize there was no natural or authentic way for me to 'lose' my eastern territory as would be historical without China really stepping up their game. A Khitan/Kara-Khitai respawn could also be another major player for the Mongols to contend with (though this might require some spawn flipzone changes). Barbarians would still play a major role in the region, to pressure the existing civs and even the Xiongnu/Xianbei.
All of the below is just a suggestion and loose outline, if you like this idea at all I fully expect you to tweak it of course! All of this is based on my (not extensive) understanding of the history of proto-Mongolic and proto-Turkic confederations in the Steppe, with UHV ideas trying to balance their history as nomadic raiders, a large steppe polity, Sinicization, their development of the first great horseback confederations, and the decentralized but expansive authority of entities like the Rouran.
Xiongnu Confederation
Spawn: 220 BC at Ordu Baliq/Otuken
Leaders: Modu Chanyu, Mugulü, Taizu (Respawn)
UU: Steppe Raider (Replaces horse archer or horseman, double gold on pillaging, double move on steppe, starts with Flanking I and II) Could possibly also have hidden nationality but at this point it risks just being an Oghuz again.
UB: Artisan Farrier (Replaces Stable, needs a better name, +2 Culture and +1

on Horse) Based on the regional and cultural history of horsemanship alongside artisan and complex saddle and bridle designs, both from a practical and aesthetic standpoint, and the key role of horses in both economy and culture
UP: Gaining a culture level spawns X free Steppe Raiders
Core: Mongolia
Historical: Tarim Basin, Manchuria, Northern China, Qinghai, Zhetsyu, Dzungaria, Ili
A variety of potential UHVs (some could be combined):
- Control X Mounted Units by 100 AD
- Acquire X gold through pillaging by 100 AD
- Acquire X Horses, X Jade, and X Sheep by 200 AD
- Control X% of territory by 300 AD
- Conquer XX number of cities by 420 AD
- Acquire X vassals by 550 AD
- Build 6 Tengris Obos, 4 Buddhist Stupas, and 2 Confucian Miaos by 600 AD
- Something to do with tribute? Like get X number of resource tributes by 550 AD--essentially a more complex version of the vassal UHV, and adding in war concessions as an option
I'm happy to help workshop names or titles for stuff if needed as well (or for other things for this modmod), or to just leave it here for your consideration. Thanks as always, Cross