I would need to check the voltage of 12V / 24V system voltage and read it remotely.
Currently there aren't any such sensors available.
Another sensor type which would be very useful is a simple wired alarm input. E.g. security cameras very often have output for wired alarm. If there was an wired input on a sensor, it would be visible on Ewelink, which would be very useful.
I doubt that this cannot be done using 433MHz frequency, so it should be a wifi device.
Multiple inputs would be very useful, e.g. input for 4 separate pairs (2x4 wires).
If all of them could read the voltage or detect if they are short or open, it would be great.
I am considering to use the door sensor as a basis for the wired input alarm device. The glass tube on the sensor, on the side where the magnet is next to (bottom of the picture), is the trigger. To trigger the alarm on the door sensor, I just shortcut the pins on each end of it and cause the alarm.
In the long run I would still want a hub of wired input alarms, which is readable by wifi and ewelink. If it could additionally / optionally read the voltage on one or all inputs, that would be even more versatile.
That door sensor could easily (by the manufacturer) be changed into hub of wired alarm input. There are 4 keys there, and one is selected (key4 in the picture) to use the alarm. There could be 3 other of those glass tubes, and connectors on each end, which would use the other keys to trigger the alarm. Sure, they wouldn't be triggered by the door alarm magnet, but a wire.
It still wouldn't measure the voltage :-(