Augmented and virtual reality experiences present significant barriers for disabled people, making it challenging to fully engage with immersive platforms. Whilst researchers have started to explore potential solutions addressing these accessibility issues, we currently lack a comprehensive understanding of research areas requiring further investigation to support the development of inclusive AR/VR systems. To address current gaps in knowledge, we led a series of multidisciplinary sandpits with relevant stakeholders (i.e., academic researchers, industry specialists, people with lived experience of disability, assistive technologists, and representatives from disability organisations, charities, and special needs educational institutions) to collaboratively explore research challenges, opportunities, and solutions. Based on insights shared by participants, we present a research agenda identifying key areas where further work is required in relation to specific forms of disability (i.e., across the spectrum of physical, visual, cognitive, and hearing impairments), including wider considerations associated with the development of more accessible immersive platforms.