I would like to see outlying, secondary cities (Rural) able to concentrate on growing food and then send some of their surplus along to older, larger, more developed cities (Urban). This might be similar to the internal trading route mechanism in Civ 5, but hopefully more nuanced.
This would be nice, giving cities without districts something to do other than building units, and if a genuine transfer mechanism would keep the outlying cities permanently smaller than the core urban cities. I think if using a transfer mechanism you would need some kind of bonus to make internal trade routes more worthwhile, like creation of gold and culture out of the trade. This could help those outlying cities slowly grow their borders even if they never build culture buildings of their own.