I don't like the names 'Town' and 'Metropolis' for the settlement levels. Especially 'Metropolis' doesn't feel to fit the theme of Elemental.
Instead of
- Outpost
- Village
- Town
- City
- Metropolis
I'd go with
- Outpost
- Village
- Township
- City
- Grand City
The original meaning of metropolis ('mothercity' from greek) is of a capital or the city of origin of a colony, not just any big city. Specifically
1 : the chief or capital city of a country, state, or region
2 : the city or state of origin of a colony (as of ancient Greece)
3 a : a city regarded as a center of a specified activity b : a large important city
Another thing is that I think it would be kind cool and provide some more personality if cities found on shores would have different scale names indicating that they're on the waterfront. For example
- Outpost
- Village
- Haven
- Port
- Grand Port
Since there are bound to be some additional buildings available for seaside cities, the levels Haven -> Grand Port could unlock these, instead of Township -> Grand City.