Addressing complex future challenges requires transdisciplinary practices. However, existing approaches for transdisciplinary collaboration tend to be limited to science-expert directions. Successful collaboration across disciplines and diverse contexts requires community agency, blurring disciplinary boundaries, and combining sciences and arts. We argue that traditional and emergent design practices provide a powerful mindset to support productive transdisciplinary collaborations for addressing complex societal problems such as climate change and social justice. Designers, historically, have struggled to translate the practices of arts and sciences into professional practice; and design can be understood as a third way of knowing that is unique from arts and sciences. Designers may use evidence, but they also generate proposals that are about preferred possibilities. We propose components of a design mindset (synthesis, modelling, speculation, facilitation, and implementation) for transdisciplinary teams to enhance futureoriented collaboration outcomes. These guidelines expand research-oriented approaches and can be used for co-designing futures in collaborative work.