In the case of city packs, it would get awkward, I agree. Since apartments were added in eco lifestyle, I think it should be possible to add them in other worlds as well from now on. Apartments are no longer exclusive to city living. But then again the city still is exclusive to city living because eco lifestyle only added apartments, not a city.
If we ever get another city world (I hope we do) it should be different enough from San mychuno so both have their own identity so each has their own value.