The idea: the city is steadily growing upward, but what if you look underground?
Add technologies in the society's branch for underground utilities:
1. Metro
2. Sewage system
3. Heating mains
4. Streams for oil
...

This can create interesting events:
1. People who do not have a home will start living underground near heating mains, becoming second-class citizens.
2. Greedy merchants who dump chemical waste and poison the population.
3. Oil pipe breaks that lead to deaths and a decline in morale.
4. Upgrading the metro will improve the arrival time of citizens to work, but in case of a breakdown, it will paralyze the city.
5. Paid access to the subway or not can increase stability or raise the budget.
...