Study the progression of African societies.

What are the final two progressions as a trading town continues to expand?

city, then territory

territory, then province

province, then city

city, then kingdom

The final two progressions as a trading town continues to expand are city, then kingdom.

Any town becomes a city once it develops enough and has the sufficient number of inhabitants. Once it grows again, it may become a kingdom - at least in the distant past this was the case.