I vote for a south american civilization.
Yes Africa need more civilization but at least there is already 3 civilization and african assets. Mesoamerica need more civilizations (and a maya campaign) but there is already 2 civilization and there is some assets. North America yes maybe.
But the worst region is South America because there is only 5 objects : the two inca UU, the two inca unique skin (castle and wonder) and the llama. This region need a DLC with many assets. The Pachacutec campaign is incas vs incas with some maya and aztec UU and aztec and maya wonders.
I have listed those assets here :
First an andean architectural set.
I think it’s possible to make civilizations with new regional units replacing the eagle warrior with another one (maybe a chasqui) add Andean maceman, bolas thrower (ranged unit who slow down unit). maybe making the slinger as a regional unit.
Mapuche can be a native civilization with cavalry with 3 unique unit : Ironwood Clubman : a footed infantry available at the castle, In the stable two unit : Bolas Rider : mounted ranged unit who slow down is opponent and Malon Raiders (reskinned steppe lancer).
Also they can access to the bolas thrower regional unit. No gunpowder unit for this civilization.
We have writhed records of their wars against the spanish in the XVI th century and this is well in the time frame of AoE2
Campaigns : Mapuche (Arauco war), New andean civilization (maybe the Chimu) and a reworked Eldorado (added in The forgotten and removed in DE) featuring the spanish.
Maybe also added an anti building ship for the american civilization and maybe a regional siege unit like here :