I have always been an avid supporter of a Sogdian civ (and I know
@Zaarin might also be interested). They were the masters of the Silk Road and pulled the strings of the most powerful empires at the time (Byzantine, Tang, Abbasids, Turk Khanate).
Unique Infrasture -
Nāf. The word originally meant "community" and refers to merchant communities and colonies that the Sogdians established throughout Central Asia.
Unique Unit -
Čākar. A type of personal retinue in the Persian world that took the form of adopted sons. It was possibly a precedent of the Mamluks. Check the
Encyclopædia Iranica page about them for more details.
My only worry is that even though we know a lot about the Sogdians, most of the info requires some decent academic research to find, and FXS is not particularly good at this unless they hire an in-house Inner Asia historian.
Not necessarily a lesser-known civ, but I would say Venetians will be a good call for Civ 7, since the game now has a city-town system and would portray polities like Venetians really well.
I saw a couple of people voted Xiongnu, and as a historian who happened to have studied the Han-Xiongnu War in detail, here are my two cents:
We do have a lot of historical information about Xiongnu, but we seriously lack some details that can make an interesting Xiongnu civ design. For instance, we don't know any proper nomenclatures in the Xiongnu language. The language itself had been lost. Most historical accounts about the Xiongnu were recorded by Chinese historians, who transliterated the words into Old Chinese (a language that also required serious reconstruction efforts).
As a result, most of the Xiongnu abilities and uniques would be strangely reconstructed words, with no uniform spelling, or just generic descriptions. Not to say that we absolutely did not have a city list for them - they just didn't have any cities (unlike Scythians who eventually had some).
Similarly, the Xiongnu UB and UU would be pretty generic, as the Chinese sources didn't record anything in particular besides the generic "royal court" 王庭, "sky altar" 祭天坛, and "(horse) archers" 骑. A Xiongnu design might not be very different from other early nomadic designs, such as the V Huns and VI Scythians.
Personally, for an early nomadic empire, I would suggest the Turkic Khanate(s) (from the early Gokturk Khanate to the Toquz-Oghuz). They were powerful and influential enough to have power plays against the Sassanids, Tang, Abbasids, and even Byzantines.
Most importantly - We know their language (
Orkhon inscriptions), we have a unique winter settlement for UB/UI (
Kishlak), we have a city list (recorded in the
Dīwān Lughāt al-Turk), we have a wonder (the inscriptions were housed in massive memorial complexes that were fully excavated), we even know what their leader looked like (
see the statue here)! That's enough info for a fully-fledged civ design, plus a non-Mongolian nomadic leader.