Designing and developing user-friendly interfaces has long been a cornerstone of HCI research. However, we are now at a turning point for how user interfaces can be designed and evaluated with new AI-based models and tools. The latest AI models have shown capabilities to model user behaviors, automate end-user tasks, and even generate user interfaces. We are at a pivotal moment to reflect on current UI design and development practices and discuss the opportunities, challenges, and risks brought by AI. Both incremental improvements and transformative opportunities exist for designers, developers, and the hand-off process in between. In this proposed workshop, we encourage participants to envision AI-enabled UI prototyping tools, workflows, and practices. By bringing together academic researchers and industry practitioners, we aim to identify opportunities to enhance prototyping tools and reshape UI creation workflows for the future as well as discuss potential negative consequences of these tools.