Notifications and as a trigger is the most basic. It would be good if the ecosystem could detect the difference between the CO portion and the Smoke portion of the detector. As a basic comment for notifications, I'd love to see different notifications, so I could setup certain things to trigger a notification that is red (like a smoke alarm) and ones that are blue (door alarm) etc etc etc or something that distinguishes them from a glance so you can quickly see what the issue is. (not an app developer SO I have no idea if that can happen
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Also, I'm not sure it is possible for alarm in the sensor to be triggered as an action as well. For example, if my detector goes off in the basement but I am in bed with the phone on silent, can it trigger the alarm in the detector in my bedroom as well. Thus replacing the need for a separate siren. I don't know that it is possible just an idea.
Also, if the battery status could trigger an alarm as well, so when the battery is down to x% notify me. As far Eldaria reported for the level of smoke, that is undoubtedly based on the detectors' abilities, would certainly be a neat addition. Sounds like a lot of implementation though.