This paper explores the integration of Causal Pathway Diagrams (CPD) into Human-Centered Design (HCD), investigating how these diagrams can enhance the early stages of the design process. A dedicated CPD plugin for the online collaborative whiteboard platform Miro was developed to streamline diagram creation and offer real-time AI-driven guidance. Through a user study with designers (N=20), we found that CPD’s branching and its emphasis on causal connections supported both divergent and convergent processes during design. CPD can also facilitate communication among stakeholders. Additionally, we found our plugin significantly reduces designers’ cognitive workload and increases their creativity in brainstorming sessions, highlighting the implications of AI-assisted tools in supporting creative work and evidence-based designs.