Hey, I think I found something really interesting. Lately, I've been doing one city only games, and each time it worked really well. Here are the pros and the cons I have noticed so far

PROS:

-Allow to focus on one city and one urban planning

-Allows to focus the defense on strategic places

-More population means more production of ressources

-All of the production bonuses gained through events go to this city

-All of the production is focused on one city, same for the ressources

-Easier to defend yourself when at war

-You finally can fill all of those tiles with your city

-You can defend better against cultural expension

CONS:

-Needs a lot of food

-Expending is quickly expensive when you aren't on a small or medium sized continent

-You can only build one thing at a time

-Can't specialize your city, even though you quickly come to a point where you have a lot of everything

That's pretty much it, if you have things to add or to say, don't hesitate to comment!