If I were choosing based on aesthetics, I would have selected earlier.
The idea here is to have what should be considered to be an "underdog" nation either by virtue of overall ability or difficulty to control or by virtue of the nations selected by others.
I have the most experience with Arcoscephale, Marignon, and T'ien C'hi, so none of those would be terribly appropriate.
I was thinking maybe Man, Agartha, Ermor, or Mictlan.