Line sketches serve as the visual DNA of fashion design, forming the essential foundation where concepts take shape, yet today's digital tools often lack the fluidity, personalization, and intelligence needed to truly support this creative process. We present FashSketch, an interactive, multimedia-driven system that reimagines fashion sketching through the lens of generative AI. Designed with a layer-based creative interface, FashSketch empowers designers to ideate, customize, and iterate on sketches seamlessly. By integrating state-of-the-art generative models, sketch-based retrieval, and large language models, the system supports advanced functionalities such as text-to-sketch generation and context-aware sketch recommendation. FashSketch not only enhances the sketching experience but also opens new multimodal pathways for creative expression, making it a powerful co-creative partner in the early stages of fashion design. Video demo is available at https://youtu.be/BX-Edz7Z7ZY.