Take a simple payroll system. Start with basic employees. Add in contract workers. And, this is the hard part, handle the exceptions - those who get paid more even though they are at the same level, those who get more vacation, those who have legacy benefits. Scale to an entire city.
Test. Because if people don't get their paychecks, you have a very angry group of customers.
Test. Because if people don't get their paychecks, you have a very angry group of customers.