With the increasing knowledge advancements and availability of mixed reality (MR), the number of its purposes and applications in vehicles rises. Mixed reality may help to increase road safety, support more immersive (non-) driving related activities, and finally enhance driving and passenger experience. MR may also be the enabling technology to increase trust and acceptance in automated vehicles and therefore help on the transition towards automated driving. Further, automated driving extends use cases of virtual reality and other immersive technologies. However, there are still a number of challenges with the use of mixed reality when applied in vehicles, and also several human factors issues need to be solved. This paper aims at presenting a research agenda for using mixed reality technology for automotive user interfaces (UIs) by identifying opportunities and challenges.