I'd actually suggest putting these as late game tile improvements.
- allow players to convert farms to suburbs (new tile improvement) / can only built on plains; grasslands, tundra and desert.
- Suburbs level up every 15 turns (same as cottages) @ lvl 5 it becomes an 'incorporated city'
- If suburb is worked, it consumes food, the more food the higher level, but produces extra science, culture and gold;
Hammers, Happiness, Tourism and faith may also be produced but perhaps it will be tied to a building or social policy
Essentially allows for 1 tile to become a super tile.
Can limit this either by making food consumption expentially higher with more suburbs worked in a city grid, or simply limit it arbitrarily as 1, 2 or 3 suburbs worked by city.