Law with two parts, only a specific type of company may own rental properties. And you may not own or be employed by more than one property; including holding stock. The same rules apply to property management companies that service land lords with few properties; possibly with larger limits on how many properties they can manage at a time.
With that basic structure we can decide how many buildings/units each company can own. For example the limit could be 100 single family homes, or 3 mid sized complexes, or 1 large tower. Then we should be able to have a system that keeps landlords from going big. Makes the system so decentralized market concepts must work and monopoly power is effectively destroyed.