It would be great if it was possible to have a websocket (client) sensor.
Binding to websockets allows instant actions, witch will come in handy for motion sensors and switches for example.
I would use it for my Xiaomi sensors connected to Deconz. Deconz offers a rest-api and websockets for specific events.
When my motion sensors detects presence this will be sent to the websocket client:
{"e":"changed","id":"2","r":"sensors","state":{"lastupdated":"2017-08-31T09:17:09","presence":false},"t":"event"} {"e":"changed","id":"2","r":"sensors","state":{"lastupdated":"2017-08-31T09:23:52","presence":true},"t":"event"}
I don’t know if it’s hard to implement, but I can imagine websockets are getting more common these days so other systems could also benefit.