By inserting the third variable, the study becomes interesting and relevant and not (yet) too complicated. I usually then ask how the mediator or moderator, pictured here, works for different conditions. These are often background variables, such as demographics. How do these relationships work for different SES levels or different child ages, for example? They might be more socio-demographic, such as different countries, different states, or different program types.
When starting to think about a research project, remember the triangle!
Baron, R. M., & Kenny, D. A. (1986). The moderator–mediator variable distinction in social psychological research: Conceptual, strategic, and statistical considerations. Journal of personality and social psychology, 51, 1173.