This paper connects with recent movements within participatory design that move beyond democracy at work and into public engagements with matters where stakes and stakeholders are uncertain. This is thus a shift from earlier participatory design projects where identifiable stakeholders were a pre-condition. To enable the becoming of stakes and stakeholders that are response-able, this paper argues for a new design space within participatory design: to enable material participation in yet unarticulated issues connected to socio-material entanglements. This becoming of stakeholder is not necessarily about solving a problem but to sensitize oneself to one's entanglements, response-ability and agency in relation to potential issues. The proposal is discussed through a design experiment centred on co-living and the messy practices of composting plastic waste in domestic settings.