Potential Eagle civ I would love to see are:
Tarascan (Purépecha) & Tlaxcalan*
Chimor & Muisca
Zapotec & Mixtec
Mississippian
* Tlaxcalan
I know that Tlaxcalan are culturally very similar for the Aztecs but their campaign will be very cool, mirroring the Montezuma campaign like the Burgundians mirrored Joan of Arc.
For Tlaxcalan I would like that they will have the Otomi as a unique unit so that civ can be used as two in one civ like the Saracen Mameluke and the Cuman Kipchak.
A lot of Otomi used to live with the Tlaxcalan but they also live in the border region between the Tarascan and Aztec empires.
The Otomi played an important role there and is very necessary to make the Tarascan campaign play smoothly, so a DLC combining Tarascan (Purépecha) & Tlaxcalan is very logical to me.