I recently started a new game of Civ5 on a giant world map marathon (with mods) with lots of free space to settle in africa ... and I noticed once again that from the look and feel of the Civ5 map I would naturally place cities in a small range of maybe 4 tiles :
city - 3 tiles empty - city - 3 tiles empty - city
For small cities below size 10 in ancient and classic era, this would look fine, would give fast access to all resource tiles and would effectively block AI from settling inside your claim. However cities placed so close have a huge overlap of about 50% territory, have all the penaltes, double upkeep, low pop, etc. and therefore in the end are less profitable. So I went with the wider spacing with 5-7 free tiles inbetween depending on geography and resources. Since small cities in Civ5BNW usually have small culture production compared to the capital with palace, wonders, etc. it takes forever for the cities to claim their 36 tiles (especially on marathon) ... the map now looks like an archipelago, most tiles between cities are not claimed. It looks unnatural. (Cities without overlap at the north african coast are Tunis (Karthago) - Tripolis - Bengasi - Port Said (Suez-canal))
I think one of the problems with tile-acquisition in Civ5 is that a city starts with one ring (6 tiles of 36 tiles) and the other 30 tiles have to be acquired one-by-one with increasing costs. (culture/cash) Civ3 and Civ4 instead used a culture range around the city, so usually already after around 10 turns(?) of culture production, a city had control of the 2nd ring / the full fat cross. Civ3 and Civ4 allowed spacing in 4-5 tiles distance without overlap.
The interesting questions for Civ6 are :
Minimum City Distance?
What happens when I place cities in 4 tiles distance with lots of overlap?
How fast will cities aquire new tiles (via culture?)?
Will there be a lot of unclaimed land between cities for a long time?
Can other players settle inbetween my citys if claim of tiles via culture is too slow?
Will cities in optimal spacing with 6-8 tiles distance look natural/good?