This paper addresses the challenge that designers face in achieving sufficient exploration, iteration, and re-examination of prior assumptions during the design process (e.g., persona creation, problem definition, solution generation, and prototyping) due to time and resource constraints. To address this, we conducted a pilot study with 15 UX practitioners, students, and instructors, and based on the results, developed StoryEnsemble, an AI-powered node-link interface tool. StoryEnsemble utilizes forward and backward propagation to support dynamic exploration and iteration throughout the design process. User studies with 10 participants demonstrated that StoryEnsemble enables rapid, multidirectional iteration and flexible movement between design stages. Ultimately, this study advances our understanding of how AI can facilitate more iterative design practices by making exploration and iteration more fluid, accessible, and engaging.