People have long talked about the merits of the Iroquois, so I wont go there again, but a wildcard civ which I want most at some point, and one I've never seen mentioned are the Haida. The closest recognition they ever got was the totem poles in BTS, but I think that they desrve more.
1) North-West coast Northern America: a gamewise empty zone
2) Recognition of the extreme diversity between North American Peoples (as distinct from the Iroquois as the Iroquois are from the Aztecs)
3) One of the longest lasting native peoples in the onslaught of European invasion
4) An artistically masterful civilization (The totam pole is just one of their iconic creations which has recognition around the world)
5) A militaristic and aggressive alternative to Monty (Often called the Vikings of North America)
6) Western Hemisphere seamanship (Canoes made from red cedar that could hold upward of 60 warriors!)
I'm not daft enough to expect them anytime soon, if at all, and I know there are numerous deserving nations, but I do believe that they would fill a unique niche in a great number of ways. Any thoughts?

Yes, Haida or Tlingit are a good idea...and i think that one that deserves not to be in is the Inuits (cover all the arctic polar circle): igloo, inuksuk...