There are a lot of civilisations we know little about, one prime example is the Huns.
There are many AoE1 civilisations that we know similarly little about.
There have been a lot of archaeological discoveries about the Mississippian cultures so we know a lot more nowadays.
The Mississippian cultures never completely disappeared, their ancestors are still around today.
I made a thread about that a while ago where I suggested 3 North American civilisations and presented some ideas of how to make them work in AoE2.