Voice interaction is a valuable method for in-vehicle interactions, especially while driving. However, voice user interfaces (UIs) exhibit drawbacks, such as insufficient feedback, which negatively impact user experience. They also lack key usability heuristics, including visibility of system status and user control. Despite the long-standing presence of head-up displays (HUDs) in vehicles, there is still significant potential for such displays to enhance user experience and contribute to the development of next-generation infotainment systems. This paper proposes a multimodal approach to enhance the usability of voice UIs in vehicles by integrating visual interfaces on the HUD. We designed three visual UIs to support voice assistants. Furthermore, we conducted an expert evaluation based on usability heuristics to assess the three UIs effectiveness. The first UI, the Baseline UI, is the simplest. The second, the Flat Fusion UI, is a conventional design that uses a scrolling method to display additional information. The third, the AR Fusion UI, is a novel interface that employs a futuristic approach, leveraging real-world depth. The findings indicate that while usability varies among the three designs, they show promising results for future research. A user study is suggested to validate this paper’s findings and to thoroughly evaluate the three UI designs.