arXiv:2610.36014v1 Announce Type: new Abstract: Head-mounted mixed reality (MR) often prioritises individual immersion, potentially drawing attention away from others sharing the room. This paper asks how MR can support co-located spatial dialogue through embodied interaction and activity visible to non-headset observers. We introduce Immersive Commons, a design framework, and Atmos Messages, a prototype combining gesture-based, ephemeral spatial messages with voice transcription, preset text, and a public floor-plan overview. Following a Research through Design approach, fifteen participants created 236 messages across two sessions, including 48 voice-authored messages. Questionnaire responses highlighted playfulness and social awareness, while observations suggested opportunities for exchange grounded in the architectural setting. Interaction clustered near the public display, although this was also where participants put on their headsets. These findings suggest the potential of ephemeral spatial messaging to support co-located exchange, with playfulness, shared visibility, and architectural context informing the design of more social and situated MR experiences.