I tried to play a 0.86 game with a single large city. I liked this way of playing, but did encounter some difficulties.

A fun part is that in order to grow to a very high city level, it is needed to build outposts to get extra grain. The difficulty however is that sometimes you are forced to have a small city somewhere on the map, but the extra grain of my outposts goes to the closest city, not the city that actually needs it. So I end up with my main city having 0 food for growth, while the small city has 300 food for growth.

Possible solution: have the extra grain go to the nearest city that can actually use it. If there is enough food already, go to the next nearest city. Alternatively, allow the player to decide where the resource goes.

Another idea: maybe add level 6 city as extra victory condition?