List of available nations:
SA:
Everything barring Columbia and Argentina
NA:
Missouri, Indiana, Arkansas
Europe:
German substates barring Brandenburg, Switzerland, Czech Republic, Hungary, Balkan countries, Russian nations, Finland
Africa:
Egypt, Latin Africa, Griqualand, Cape Republic
India:
Everything barring East India and Bengal
SEA:
Everything
China:
Everything barring DRC, Canton, and Tibet
Rest of Asia:
Everything
Personally, I'd recommend Persia, a Russian state, a Balkan state or Hungary, or Siam or Nusantara, with the recommendations being based on regions where there isn't a local regional power.