Recently played with a very interesting mod that made it so settlers cost 1,000 production, but you got one for free every time you researched a new technology from an era. It was an interesting game. I do think investing in new cities is too valuable. Perhaps of there was another type of district you could build that would essentially be another City center that brings all of its benefits and extended the range of the city's workable tiles could be explored. But that feels out of the scope of civ 6.