An important historic correction, with the Mississippian, there were only a handful know instances of human sacrifice. It did happen, very rarely. Only Mound 72 of Cahokia has clear evidence of mass human sacrifice.
Also strontium analysis on the teeth showed that they were mostly local people. So sacrifice would not have been a big motivation to go to war for the Mississippians.
So I think it is bad to make that a big focus of a civ design on something that most likely only happend once, in one city.
Beside that, I would love to see the Mississippian sometime in game. The more I learn about them, the more I feel like they deserve a spot in AoE II.
(If you want you can have a look at the sources I have used in my Mississippian design)