8) Thai or Cambodia (Khmer)
There won't be a "true" Thai civ while "Siam" as it is now is in the game - Ramkhamhaeng is a Thai national icon so it would be odd for him to lead a separate civ, plus Siam is an archaic name for Thailand (not the game's Sukothai Kingdom) and both the UU and UB are Thai.
Why are people naming single states of America such as Texas or Mississippi?
"Mississippian" is the Western name of an artefact culture first associated with sites along the Mississippi River - it has nothing to do with the state. It's the native culture famous for mound-building; claims that some of these sites were as large as European cities are quite possibly exaggerations, although it is undoubtedly true that they were larger than any American city until post-independence. The largest mound "city", Cahokia, is name-checked in Civ V as a city-state (and while I don't think a Mississippian "civ" makes any sense, I was thrilled to see Cahokia in).
Problems with including the Mississipians as a civ are that there's nothing known about them, and it's not at all clear they were anything resembling a unified society - it's an archaeological designation, like "Clovis", and the mound architecture doesn't even feature in Native American oral histories or mythology as far as I'm aware. There are no known Mississippian leaders, nothing that could be used as a unique unit, no information on elements of their culture that could inspire a UA.
If that's supposed to be joke ok but I don't see why they'd seriously do that.
Inuit's presumably supposed to be a joke, but that's in the list. It even has a vote.
If we're going to add more Asian civs (which I wouldn't mind in the least) we should add some historically more significant ones than the two you mentioned.
The Champa lasted over a millennium, the longest-surviving civ in Southeast Asia. I'd argue that that makes Vietnam historically somewhat significant. Tibet's empire was extremely large and lasted about three centuries as an essentially Tibetan-led state. The Khmer are better-known, but of the remaining Asian candidates (excluding Mesopotamia), it's not clear that there are any that are "more significant" and not already represented in some form (although arguably China represents Tibet, since both were periodically under Mongol rule, China was under Tibetan rule at times, and Tibet under Chinese rule at others - it's actually not straightforward to delineate "Tibet" from "China" through most of the states' history).