Maybe a property tax adjustment based on how much time the property has spent vacant over the past N years?

Something like, every year the property owner pays:

    property value * (average vacancy % over N years)

If your property is occupied, you pay nothing extra. If your property sits idle for one year out of five, you pay 20% the property's value to the city every year until your rolling average improves. Maybe throw in some incentives like a partial rebate when the owner signs a new tenant after a stretch of vacancy.