Study theories connected with the topic. Explore when these theories would hold and when they do not hold. Try the revisionist approach by attempting to prove the opposite of accepted theories. Try the Falsificationist approach. Look for that evidence in the real world.
Replication is useful for learning research tools and the strength of theories.
Try testing the theory for new geographic regions. Perhaps there is no study of the Consumption Function in Thailand. Try testing the theory for new periods of time. Try measuring the theoretical variables in different ways. There are many ways of measuring income.